What Happens When a Contract Is Based on a Mistake?

Contracts are built on the assumption that both parties know what they are agreeing to. But what happens when one or both of them are wrong? What happens when the subject of the contract does not actually exist, or when the written agreement does not reflect what the parties actually negotiated, or when one party signs a document without understanding what it says?

These situations fall under the legal doctrine of mistake. In Canadian contract law, a mistake can, in certain circumstances, render a contract void (as if it never existed), voidable (capable of being set aside), or subject to correction through a court order. The consequences of a mistake depend on the type of mistake, who made it, how significant it was, and whether the other party knew about it or contributed to it.

This guide explains how the law treats mistakes in the contracting process. It covers the major categories of mistake recognized in Canadian law, the remedies available when a mistake is established, and the practical considerations that determine whether a court will intervene. If you are involved in a commercial dispute where a mistake may have been made in the formation of a contract, understanding these principles is the essential first step.

What Is a “Mistake” in Contract Law?

A mistake in contract law is not the same as a bad bargain. If you agree to pay too much for something, or if the deal turns out to be less profitable than you expected, that is not a legal mistake. The law of mistake is concerned with situations where one or both parties held a false belief about a material fact at the time the contract was formed, and that false belief affected the nature of their agreement in a fundamental way.

As Thompson J. cautioned in McMaster University v. Wilchar Construction Ltd., the law of mistake is “a difficult and elusive topic” in which “the principles upon which the Courts will intervene and the circumstances in which they will do so have not been precisely settled.” The decided cases are, in the court’s words, “open to a number of varying interpretations and are difficult to reconcile.”

This difficulty arises because “mistake” is not really a single legal doctrine. It is a family of related doctrines, each with its own rules, requirements, and consequences. The main branches of this family, as recognized in Canadian law, include mistake as to the terms of the contract, mistake as to the written record (addressed through rectification), mistake as to fundamental assumptions about the subject of the contract, mistake as to the identity of the other contracting party, and the ancient doctrine of non est factum, which applies when a person signs a document fundamentally different from what they believed it to be.

Who Is Mistaken? Common, Mutual, and Unilateral Mistake

One of the most important questions in any mistake case is who was mistaken and how that mistake relates to the other party. Canadian law recognizes three categories based on who holds the false belief.

Common Mistake

A common mistake (sometimes called a “shared” mistake) occurs when both parties to the contract hold the same false belief about a material fact. For example, both the buyer and seller believe the painting being sold is an original, when it is actually a reproduction. Or both parties enter into a contract for the sale of goods that, unknown to either of them, have already been destroyed. In a common mistake, both parties are operating under the same misapprehension. The contract may be void at common law if the mistake goes to the very root of the agreement, or it may be voidable in equity if the mistake, while serious, does not entirely eliminate the subject of the contract.

Mutual Mistake

A mutual mistake is different. Here, both parties are mistaken, but they hold different misapprehensions. One party thinks the contract is about one thing, while the other party thinks it is about something else. Neither party’s interpretation is clearly right or clearly wrong. This situation is closely related to issues of offer and acceptance: if the parties never actually agreed on the same terms, there may never have been a valid contract in the first place.

Unilateral Mistake

A unilateral mistake occurs when only one party is mistaken and the other party either knows the truth or is not mistaken at all. This is the most common and most litigated type of mistake. The general rule is that a unilateral mistake, standing alone, will not affect the validity of a contract. If I agree to certain terms and later realize I misunderstood them, that is generally my problem. As James L.J. stated in Tamplin v. James: “If a man will not take reasonable care to ascertain what he is buying, he must take the consequences.”

However, a unilateral mistake can become legally relevant when the non-mistaken party knew or ought to have known about the other’s mistake and took unfair advantage of it. In such cases, the law is concerned not just with the mistake itself, but with the unconscionability of allowing one party to benefit from the other’s error. As the court held in Milinx Marketing Group Inc. v. ABC Inc., a party seeking relief for unilateral mistake must show that the other side “unconscionably took advantage” of the mistake. Where both parties were represented by counsel and the agreement fully reflected the parties’ intentions at the time, relief will not be available simply because one side later realized it made a poor deal.

The Consequences of Mistake: Void, Voidable, and More

The legal consequences of a mistake depend on how the mistake is characterized and whether it is addressed at common law or in equity.

Void at Common Law

A contract that is void for mistake is treated as if it never existed. No rights or obligations arise from it. This is the most drastic result and is reserved for the most fundamental mistakes, such as where the subject of the contract does not exist or where the mistake goes to the very root of the agreement. The void result is significant because it means that even third parties who acquired rights under the contract (for example, a purchaser who bought goods from the original buyer) cannot enforce those rights. The contract simply never was.

Voidable in Equity

A contract that is voidable for mistake exists and is valid unless and until the mistaken party elects to have it set aside. This is the more common equitable result. It protects third parties because the contract remains valid until it is formally rescinded, and courts will generally decline to set aside a contract where doing so would prejudice an innocent third party who relied on the contract’s existence. Rescission is an election: once the mistaken party affirms the contract or takes steps that are inconsistent with setting it aside, the right to rescind may be lost. The party seeking rescission must also be able to make restitution, meaning they must be able to return whatever benefits they received under the contract.

Other Remedies

Courts may also refuse to grant equitable remedies like specific performance where a mistake has been made, even if the contract is not formally set aside. And, as discussed below, rectification allows a court to correct the written record of the contract to reflect what the parties actually agreed, without eliminating the contract itself.

Non Est Factum: “It Is Not My Deed”

One of the oldest and most straightforward branches of the law of mistake is the doctrine of non est factum. The Latin phrase means, roughly, “that is not my doing.” It applies where a person has signed a written contract but claims that the document they signed was fundamentally and radically different from what they believed it to be.

Unlike the other branches of mistake, which typically make a contract voidable, non est factum renders the contract void. This makes it a powerful defence, but courts have set a high bar for its use.

The Three Elements

The formulation set out by Glube J. in Castle Building Centres Group Ltd. v. Da Ros captures the requirements concisely. First, the burden of proving non est factum rests on the party seeking to disown their signature, and it is a heavy burden when the person is of full capacity. Second, the person must show that the document signed was radically or fundamentally different from what they believed they were signing. Third, the person raising the plea must not have been careless in taking reasonable measures to inform themselves about the contents and effect of the document before signing it.

Carelessness as a Bar

The third element is where most non est factum claims fail. A person who could have read the contract but chose not to, or who signed without asking questions, will generally be held to what they signed. As Gerein J. observed in Bank of Montreal v. McIntosh, non est factum applies where “a person’s mind did not go with his or her hand.” But if the reason the mind did not go with the hand is that the person simply could not be bothered to read what they were signing, the defence will not succeed.

Courts have been particularly strict in applying this requirement. In Guarantee Co. of North America v. Ciro Excavation & Grading Ltd., the Ontario Court of Appeal found that the absence of a misrepresentation and the party’s carelessness were “fatal” to her non est factum defence. She had never taken steps to inform herself about what she was signing and was, in the court’s view, “careless to the point of indifference.”

Relationship to Misrepresentation

In practice, most cases of non est factum involve a misrepresentation by the other party or a third party about the nature of the document being signed. There is recent Ontario authority, including from the Court of Appeal in Bulut v. Carter, that a misrepresentation is actually a prerequisite to establishing non est factum. In Bulut, the court stated: “The defence of non est factum is available to someone who, as a result of misrepresentation, has signed a document mistaken as to its nature and character and who has not been careless in doing so.”

This creates a hierarchical relationship with misrepresentation. If non est factum applies, the contract is void. If it does not apply but there was a misrepresentation, the contract may still be voidable on the basis of misrepresentation or subject to rectification. As the court held in Howatson v. Webb, the contract is not void merely because it omits or misstates a material clause; in such a case, the contract may be voidable or subject to rectification, but it is not void.

Mistake as to Terms

A mistake as to the terms of a contract arises when one or both parties are mistaken about what obligations the contract actually contains. This can happen for a variety of reasons: one party may believe a particular warranty is included when it is not, or may have a mistaken belief about the scope of an exclusion clause, or may believe a statute implies a term into the contract when no such term exists.

The law’s response to a mistake as to terms depends on whether the mistake is common, mutual, or unilateral.

Common Mistake as to Terms

Where both parties share the same mistaken belief about a term, they often do not need the law to correct the problem. If both sides agree the term was supposed to be X and the contract says Y, the parties will ordinarily fix it themselves. The issue arises where one party later refuses to acknowledge the shared mistake, usually because the “mistaken” version of the contract turns out to be more favourable. In such cases, the remedy is usually rectification, discussed below.

Unilateral Mistake as to Terms

The more contentious area involves unilateral mistake. The general rule is that a party cannot escape a contract simply by saying they meant something different from what the objectively-determined terms provide. If, viewed objectively, the parties agreed on certain terms, there is a contract on those terms, regardless of one party’s subjective misunderstanding.

The exception arises where the non-mistaken party knew or ought to have known that the other party was operating under a mistake about the terms. In such cases, it is unconscionable for the non-mistaken party to hold the mistaken party to a bargain the non-mistaken party knew was not what the mistaken party intended. As the court held in Jardine v. Mann, for unilateral mistake to provide relief, the mistaken party “must show a mistake with respect to the agreement and that the sellers knew of his mistake.”

Mistake as to Terms versus Mistake as to Effect

An important distinction exists between a mistake about the actual terms of a contract and a mistake about the commercial consequences or effect of those terms. The bulk of authority holds that a mistake as to the effect or consequences of a contract is not operative at all as a basis for relief. In Clarion Ltd. v. National Provident Institution, the court held that equity will not grant relief where the mistake goes not to the terms of the arrangement but only to its commercial consequences and effect. As Rimer J. stated, “it is ordinarily no part of equity’s function to allow those who do make such bargains to escape from them.” A party who enters into a contract without fully understanding its commercial implications has made a bad bargain, not a legal mistake.

Rectification: Correcting the Written Record

Rectification is one of the most frequently argued areas of the law of mistake. It is also one of the most practically important for commercial parties, because it does not eliminate a contract but rather corrects it to reflect what the parties actually agreed to.

What Rectification Does (and Does Not Do)

As Brown J. explained for the majority of the Supreme Court of Canada in Canada (Attorney General) v. Fairmont Hotels Inc., rectification allows a court to fix a written instrument that has “incorrectly recorded the parties’ antecedent agreement.” Where a term has been omitted, an unwanted term included, or a term incorrectly expressed, the court can rectify the instrument so that it accords with the parties’ true agreement.

Critically, however, rectification is “limited solely to cases where a written instrument has incorrectly recorded the parties’ antecedent agreement.” It is not available where the basis for seeking it is that one or both parties wish to amend the agreement itself. As the court stated in Performance Industries Ltd. v. Sylvan Lake Golf & Tennis Club Ltd., “The court’s task in a rectification case is to restore the parties to their original bargain, not to rectify a belatedly recognized error of judgment by one party or the other.”

In other words, courts rectify instruments, not contracts. As James V.-C. stated in the foundational case of Mackenzie v. Coulson: “Courts of Equity do not rectify contracts; they may and do rectify instruments purporting to have been made in pursuance of the terms of contracts.”

Rectification for Common Mistake

The requirements for rectification based on a common mistake were set out by Brooke J.A. in H.F. Clarke Ltd. v. Thermidaire Corp. The party seeking rectification must satisfy the court that the parties were in complete agreement on the terms of their contract but wrote them down incorrectly. There must be a prior agreement between the parties, objectively determined, with terms that are definite and ascertainable, that was still effective at the time the instrument was executed. The instrument must fail to accurately record that agreement, and the error must be in the record itself, not in the parties’ judgment about consequences. Finally, if the instrument were rectified, it would accurately reflect the prior agreement.

Rectification for Unilateral Mistake

Rectification is also available for unilateral mistake, though the requirements are more stringent. Following the Supreme Court of Canada’s decision in Performance Industries Ltd. v. Sylvan Lake Golf & Tennis Club Ltd., the party seeking rectification must show: the existence and content of a prior oral agreement; that the written record does not correspond to the oral agreement; that the non-mistaken party knew the written record did not match the oral agreement (or, in some formulations, was guilty of “fraud” in the equitable sense); and the precise form to which the contract should be rectified. The standard of proof is high: the party must provide “convincing proof” that the written record is wrong.

Relationship to the Parol Evidence Rule

Rectification operates as an exception to the parol evidence rule and the signature doctrine. The parol evidence rule holds that where a contract has been reduced to writing, oral evidence cannot be admitted to prove that a particular term was verbally agreed but omitted from the written agreement. The signature doctrine, from L’Estrange v. F. Graucob, Ltd., holds that a party who signs a document is bound by its terms whether they read it or not, absent fraud or misrepresentation. Rectification allows a court to look behind the written record and consider extrinsic evidence to determine what the parties actually agreed, even though the written record says something different.

Equitable Considerations in Rectification

Because rectification is an equitable remedy, it is subject to the usual equitable defences, including clean hands (the party seeking rectification must not have acted inequitably), delay and acquiescence (a party who waits too long may lose the right to seek rectification), and prejudice to third parties (a court will be reluctant to rectify a contract where doing so would harm an innocent third party who relied on the written record).

Mistaken Assumptions: When the Background Is Wrong

A mistake in contracting need not relate to the terms of the contract at all. Instead, one or both parties may have entered into the contract under a mistaken assumption about the context or background against which they were contracting.

This is a distinct area of the law, and an area where the courts have been particularly cautious about granting relief. The reason is straightforward: contracts are about allocating risk. Every contract involves some degree of uncertainty, and each party is expected to take reasonable steps to inform itself before agreeing to the deal. If a party could escape a contract every time a background assumption turned out to be wrong, the certainty that contracts are meant to provide would be fundamentally undermined. As Cote J.A. stated in Radhakrishnan v. University of Calgary Faculty Assn., contracts “could not be set aside because one of the parties had defective information with which to allocate risk.”

Lord Atkin’s Three Categories

The classic statement of the types of mistaken assumptions that can affect a contract comes from Lord Atkin in the House of Lords’ decision in Bell v. Lever Brothers Ltd. Lord Atkin identified three categories of mistaken assumption that could, in principle, have legal consequences: mistake as to the identity of the contracting parties; mistake as to the existence or possibility of the subject of the contract, including where the subject has been destroyed before the contract was formed, or where a purchaser purports to buy what they already own; and mistake as to a quality of the subject that is essential, where the thing without that quality is “essentially different from the thing as it was believed to be.”

To these categories, the English Court of Appeal in Solle v. Butcher added a possible fourth: a mistake as to fact or rights that is fundamental, where the party seeking relief is not at fault for the mistake. In Canadian law, this fourth category is generally treated as an extension of the third.

Mistake as to Existence of the Subject

The most straightforward case of a mistaken assumption arises where the subject of the contract does not exist at the time the contract is formed. If both the buyer and the seller believe they are contracting for the sale of a specific cargo of goods, and that cargo has already been destroyed by fire before the date of the contract, the contract may be void for common mistake. This principle finds statutory expression in the Sale of Goods Act, which provides that where there is a contract for the sale of specific goods and the goods, without the knowledge of the seller, have perished at the time the contract is made, the contract is void.

Mistake as to an Essential Quality

Mistake as to the quality of the subject is where the law becomes most difficult. Following Bell v. Lever Brothers, the threshold is exceptionally high. The mistake must be about a quality that is so fundamental that the thing contracted for, without that quality, is “essentially different” from what it was believed to be. A mistake about value, or about the commercial desirability of the subject, is generally not enough. If you buy a painting believing it to be more valuable than it is, that is a bad bargain, not a legally operative mistake.

Common versus Unilateral Mistaken Assumptions

For a mistaken assumption to affect the contract, it must generally be a common mistake, shared by both parties. A unilateral mistaken assumption about the background or context will ordinarily have no effect on the validity of the contract, because the other party is innocent and should not be made to bear the consequences of the mistaken party’s error. The exception, again, arises where the non-mistaken party knew of the mistake and took unconscionable advantage of it.

Mistake as to Identity

A distinct category of mistaken assumption involves the identity of the other contracting party. The law in this area has been described as plagued by “fine distinctions and case law which is sometimes not easy to reconcile.” There is surprisingly little Canadian authority, and what exists tends to follow English precedents.

How Identity Mistakes Typically Arise

A mistake as to identity usually arises because one party has been misled about the other’s identity, often by a fraudster or “rogue” who impersonates a real person. The classic scenario involves a rogue who obtains goods on credit using a stolen identity, then sells those goods to an innocent third party and disappears. The question becomes whether the original seller has a contract with the rogue (voidable for misrepresentation, meaning the third party may be protected) or no contract at all (void for mistake, meaning the third party has no rights).

This tension between protecting the original seller and protecting the innocent third-party purchaser is at the heart of the law of mistaken identity. The House of Lords addressed it extensively in Shogun Finance Ltd. v. Hudson, a case in which a rogue obtained a car on credit using the identity of a person named Patel and then resold it to Hudson. The court held that Shogun’s contract was with Patel (whose identity was in the written agreement) and that, since Patel never agreed to the contract, the contract was void for mistake.

The Significance of the Void/Voidable Distinction

The distinction between void and voidable is critical in identity cases because it determines who bears the loss. If the contract is void, the original owner retains title to the goods and can recover them from the third-party purchaser, who is left with a claim (usually worthless) against the disappeared rogue. If the contract is merely voidable for misrepresentation, the third party who acquired the goods before the contract was avoided may have good title. The law has struggled to find a principled basis for drawing this line, and the case law in this area reflects that difficulty.

Carelessness and Fault

As in the non est factum context, a party who was careless about the identity of the person they were dealing with will have difficulty claiming relief for mistaken identity. In Heydary Hamilton PC v. Bay St. Document Systems Inc., the court found that a law firm seeking to rescind a lease for mistaken identity had been “sloppy in not asking more questions and not insisting that he had a legible copy of the confusing illegible documentation to read before he signed it.” Carelessness in verifying the identity of the other party undermines the case for relief.

Equitable Defences and Bars to Relief

Because many of the remedies for mistake are equitable in nature, a party seeking relief must satisfy the court that the equitable considerations favour intervention. Several well-established defences can prevent a mistaken party from obtaining relief even where a mistake has been established.

Clean Hands

The party seeking equitable relief must come to the court with clean hands. A party who has itself acted inequitably, for example by misleading the other party or concealing relevant information, may be denied relief for its own mistake.

Affirmation and Delay

A party who discovers a mistake but continues to perform under the contract, or who delays unreasonably before seeking to have the contract set aside, may be taken to have affirmed the contract and lost the right to rescind. The doctrine of laches applies: equity aids the vigilant, not the indolent.

Restitution

A party seeking to set aside a contract for mistake must generally be able to restore the other party to its pre-contractual position. If restitution is impossible, for example because the subject of the contract has been consumed, altered, or transferred to a third party, rescission may not be available.

Prejudice to Third Parties

Courts will generally decline to set aside a contract where doing so would prejudice the interests of an innocent third party who acquired rights under the contract in good faith. This is a particular concern with the void/voidable distinction: a contract that is void from the outset affects third parties who may have relied on its existence, while a voidable contract can be set aside only before a third party’s rights have crystallized.

Hardship

Even where a mistake is established, a court may decline to grant relief if doing so would cause disproportionate hardship to the non-mistaken party. Equity acts to prevent unconscionable outcomes, and if setting aside the contract would itself produce an unconscionable result, the court may leave the parties where they are.

Mistake and Related Doctrines

The law of mistake does not exist in isolation. It overlaps with several related doctrines, and understanding those relationships is important for anyone evaluating a potential mistake claim.

Misrepresentation

There is significant overlap between mistake and misrepresentation. Where a party’s mistake was caused by a false statement made by the other party, both misrepresentation and mistake may apply. In practice, misrepresentation is often the more predictable basis for a claim, because the law of misrepresentation is better settled and the requirements are clearer. The key difference in terms of consequences is that misrepresentation makes a contract voidable, while certain types of mistake (non est factum, common mistake at common law) can make it void.

Frustration

Mistake and frustration are sometimes described as two sides of the same coin. Mistake relates to a false belief about the world as it existed at the time the contract was formed. Frustration relates to an unexpected change in the world after the contract was formed. Both can render performance impossible, but the timing of the event or discovery determines which doctrine applies. One important difference in consequence is that a frustrated contract is not void from the outset but is brought to a prospective end.

Estoppel

Where mistake operates to deny that the truth is the truth (and thereby avoids a contract), estoppel operates in the opposite direction: it holds the parties to the mistake as though it were the truth. A party who has represented a particular state of affairs and induced the other party to rely on that representation may be estopped from later denying it, even if the representation was mistaken.

Good Faith and Honest Performance

Since the Supreme Court of Canada’s decision in Bhasin v. Hrynew, Canadian contract law has recognized a general organizing principle of good faith in contractual performance, including a duty of honest performance. Where a non-mistaken party’s knowledge of the other’s mistake intersects with the duty of honesty, the obligation to act in good faith may reinforce the equitable basis for relief.

Unconscionability and “Snapping Up”

The doctrine of unconscionability intersects with mistake where one party takes advantage of the other’s error. The concept of “snapping up” arises where a party, knowing that the other party has made a mistake, rushes to accept the mistaken offer before the other party can correct it. Where a party “snaps up” an offer they know to be mistaken, a court may refuse to enforce the contract or set it aside.

Bidding Contexts and Contract A/Contract B Analysis

One practical area where mistakes arise with some frequency is in the tendering and bidding process. A bidder who makes a computational error in preparing a tender may submit a price that does not reflect its actual costs. This can raise issues under both the law of mistake and the law of tenders.

In the Contract A/Contract B framework recognized in Canadian procurement law, the submission of a compliant bid creates a preliminary contract (Contract A) between the bidder and the party calling for tenders. A mistake in the bid can raise the question of whether Contract A was ever validly formed, or whether the bidder is entitled to relief from the consequences of its mistake. The law in this area involves a tension between the integrity of the tendering process and the unfairness of holding a bidder to a price it never intended to offer.

The leading case in this area is McMaster University v. Wilchar Construction Ltd., where the court considered whether a contractor that had made a computational error in its bid was entitled to withdraw. The court acknowledged the difficulty of the law of mistake in this context and applied principles of unconscionability and unilateral mistake, recognizing that it would be inequitable to hold a bidder to a price that the owner knew or ought to have known was the product of an error.

Evidentiary Challenges in Mistake Cases

One of the most significant practical obstacles in any mistake case is proof. Mistakes, by their nature, relate to what parties believed or understood at the time of contracting. This is a subjective matter, and courts are understandably cautious about accepting after-the-fact claims that a party’s understanding differed from what the written record shows.

The standard of proof for mistake is the civil standard, the balance of probabilities, but courts have consistently emphasized that the evidence must be “convincing.” In rectification cases in particular, the party seeking to alter a written record bears a heavy evidentiary burden. As the law requires proof of a prior agreement that differs from the written instrument, there must be reliable evidence of what that prior agreement contained. Where the negotiations were not documented, or where significant time has passed, establishing that evidence can be extremely difficult.

Courts have also stressed the importance of timely action. A party that discovers a potential mistake but waits months or years to assert it may face both evidentiary and equitable obstacles.

Key Legislation

Although the law of mistake in Canada is primarily judge-made, certain statutes are relevant. The Sale of Goods Act (in Ontario and other common law provinces) codifies specific rules relating to mistakes about the existence or ownership of goods being sold. The Canada Business Corporations Act and its provincial equivalents contain provisions about the indoor management rule, which can be relevant in cases of mistaken identity involving corporate parties. And the equitable jurisdiction of the superior courts, inherited from the English Court of Chancery, provides the foundation for remedies such as rectification, rescission, and the refusal of specific performance.

Conclusion

The law of mistake in Canadian contract law is complex, uncertain in parts, and built on centuries of case law that courts themselves acknowledge is difficult to reconcile. But the underlying principles are grounded in common sense. A contract requires genuine agreement. Where that agreement is undermined by a fundamental misapprehension, whether about what the contract says, what its written record provides, or the very existence of what is being contracted for, the law provides mechanisms to correct the injustice.

At the same time, the law is cautious about intervening. Contracts allocate risk, and parties are expected to exercise reasonable care before signing. A bad bargain is not a mistake. Carelessness is not a defence. And the interests of innocent third parties must always be weighed against the interests of the mistaken party.

For anyone facing a contract dispute where a mistake may be in play, the critical first step is to identify the type of mistake, determine who was mistaken and what they were mistaken about, and assess whether the available remedies fit the facts. The law of mistake may be complicated, but it exists for a reason: to ensure that contracts are built on truth, not error.