Mistake

Mistake is a narrow doctrine. It does not rescue a party from imprudence, disappointment, market movement, poor due diligence, or the discovery that a transaction is less advantageous than expected. Its central question is more fundamental: was there, in law, an agreement at all, or has the apparent contract been vitiated because the parties, or one of them, proceeded on an error so basic that the law refuses to treat the transaction as binding?

For Durham first-year Contract, mistake sits at an important point in the architecture of the module. Weeks 1 and 2 asked whether agreement was formed: offer, acceptance, certainty, and intention. Weeks 4 and 5 asked what the contract contains and how far its terms may exclude liability. Week 6, misrepresentation, supplied a principal route of escape where one party has been induced by another party's false statement. Mistake is different. It usually concerns erroneous assumptions not attributable to an actionable representation, and its ordinary effect is not rescission of a valid contract but voidness: if the plea succeeds, the contract is treated as never having come into existence.

That consequence explains both the attraction and the danger of the doctrine. Voidness may defeat third-party rights. In the classic identity cases, the original owner seeks to recover goods from an innocent purchaser by arguing that the rogue acquired no title because the contract with the owner was void for mistake. The law must then choose between two innocent parties. The strictness of the doctrine reflects this distributive problem. A broad doctrine of mistake would destabilise transactions and undermine commercial risk allocation.

The principal categories are common mistake, mutual mistake, and unilateral mistake. A common mistake arises where both parties share the same erroneous assumption. The leading modern authority is The Great Peace, which states a stringent common law test and rejects a wider equitable jurisdiction to set aside contracts for common mistake. A mutual mistake arises where the parties are at cross-purposes and neither understanding can objectively prevail. A unilateral mistake arises where only one party is mistaken, and the other knows, or is treated as knowing, of that mistake; within this category, mistake as to identity has produced the most difficult cases.

The governing method is objective. Contract law does not simply ask what the parties privately thought. It asks what their words and conduct, judged objectively, communicated. If an objective agreement can be found, the mistaken party is normally bound unless the mistake concerns an essential matter and the other party's knowledge, or the nature of the transaction, prevents the law from finding true consent. The exam discipline is therefore precise: identify the type of mistake; ask whether the issue is formation, construction, misrepresentation, frustration, rectification, or risk allocation; state the legal consequence; then apply the cases without turning mistake into a general fairness jurisdiction.

The history of mistake is a history of constraint. Nineteenth-century contract law developed under the influence of will theory, but the courts increasingly preferred an objective account of agreement. A party's private intention was not enough. The law looked to outward manifestation. This is visible in Smith v Hughes, where the buyer's private belief about oats did not prevent a contract if the seller had not promised what the buyer assumed. That case remains a useful warning: mistake is not a doctrine for correcting unilateral assumptions about quality or motive.

The older cases often used the language of consensus ad idem, but that expression can mislead. It suggests that contract depends upon actual psychological meeting of minds. Modern law is more exacting and more practical. The apparent absence of actual consensus matters only where, objectively, no agreement can be identified or where the law treats the mistake as going to an essential assumption of the transaction. Raffles v Wichelhaus is the orthodox example: the contract referred to cotton to arrive on the ship Peerless, but there were two ships of that name. If neither party's meaning could objectively prevail, there was no contract because the agreement was incurably ambiguous.

Common mistake developed through cases concerning the existence of the subject matter or the legal possibility of the promised exchange. Couturier v Hastie and section 6 of the Sale of Goods Act 1979 show the strict position where specific goods have already perished without the parties' knowledge. Cooper v Phibbs illustrates a different but related pattern: a party contracted to take a lease of a fishery when, in truth, he already had a beneficial interest in it. The transaction rested on a false legal assumption so fundamental that equity intervened. These cases show the older law's concern with transactions that, in substance, could not sensibly operate as the parties supposed.

The twentieth century produced two competing tendencies. Bell v Lever Bros set a high threshold for common mistake. The House of Lords refused to void compensation agreements merely because the employer later discovered that the employees could have been dismissed without compensation. The mistake affected the value and prudence of the bargain, but it did not make the subject matter essentially different. Later, Denning LJ in Solle v Butcher attempted to preserve a broader equitable power to set aside contracts where the common mistake was fundamental and enforcement would be unconscionable. That approach softened the harshness of Bell and allowed terms to be imposed on rescission.

The Court of Appeal rejected that dual-track approach in The Great Peace. It held that English law recognises no separate equitable jurisdiction to rescind for common mistake where the common law test is not met. The decision restored doctrinal austerity: if the common law contract is valid, equity cannot undo it merely because the mistake was serious. The case aligns mistake with commercial certainty and risk allocation.

Unilateral mistake, especially mistake as to identity, has followed a separate and more troubled path. Cundy v Lindsay favoured the original owner where a rogue obtained goods by pretending to be a reputable firm. Phillips v Brooks and Lewis v Averay, by contrast, tended to protect the innocent third-party purchaser in face-to-face transactions by treating the seller as intending to contract with the person physically present. Shogun Finance reasserted a distinction between written and face-to-face dealings, holding by majority that a finance company contracted only with the named person in the written documents, not with the rogue. The result is doctrinally defensible but commercially uncomfortable: form can determine loss allocation.

For exam purposes, this history matters because it explains the modern law's suspicion of mistake. The doctrine remains available, but only at the margins of contractual consent. The more ordinary remedial routes are construction, implied terms, misrepresentation, statutory regulation, frustration, or breach. A strong answer recognises that mistake is not an invitation to do justice at large; it is a controlled set of rules protecting the integrity of agreement while preserving security of transactions.

The first principle is classification. Do not write about mistake as a single undifferentiated doctrine. Common mistake concerns an erroneous assumption shared by both parties. Mutual mistake concerns cross-purposes: A means one thing, B means another, and the issue is whether the objective approach can select one meaning. Unilateral mistake concerns one party's error, coupled with circumstances that prevent the other from insisting on the apparent agreement. Each category has different requirements and different policy implications.

The second principle is objectivity. Contract law begins with outward manifestation, not private mental reservation. If a reasonable person would conclude that the parties agreed on particular terms, a party who privately misunderstood will usually be bound. This follows from the same discipline encountered in offer and acceptance. Mistake is not a licence to reopen formation simply because one party later says that subjective consent was absent. The question is whether, applying the objective test, the law can identify a bargain and whether the relevant mistake is sufficiently fundamental to negative the contract.

Common mistake is the most important category for problem questions involving subject matter, title, possibility, or assumptions about legal or factual states of affairs. The modern test comes from The Great Peace. The parties must have contracted on the basis of a shared assumption; the assumption must be fundamental; the mistake must not be attributable to the fault of the party seeking relief; the contract must not allocate the risk of the relevant state of affairs to that party; and performance, in the relevant sense, must be impossible or the subject matter essentially different from what was contemplated. The emphasis is not mere hardship or serious financial disadvantage. The question is whether the state of affairs is so radically different that the contract cannot fairly be treated as the contract the parties made.

The landmark cases are best read as a map of limits.

Smith v Hughes is the starting point for the objective approach. The buyer thought he was buying old oats; the seller supplied new oats. The case is not simply about oats. It establishes that a party's uncommunicated assumption does not define the contract. If the seller did not promise old oats and the buyer objectively agreed to buy the oats offered, the buyer's private mistake is irrelevant. The case therefore belongs with Week 1 and Week 2 as much as with mistake: apparent assent normally binds.

Couturier v Hastie represents mistake as to the existence of subject matter. A cargo of corn had been sold before the contract because it was deteriorating, and the House of Lords held that the buyer was not liable for the price. The parties had proceeded on the assumption that there was an existing cargo to be sold. Where the assumed subject matter does not exist, the apparent bargain may fail. Section 6 of the Sale of Goods Act now supplies a statutory rule for specific goods that have perished at the time of contracting.

Bell v Lever Bros is the leading authority restraining common mistake. The compensation agreements were not void even though both parties were unaware that the employees' misconduct gave the employer a right to dismiss them without compensation. The case demonstrates the law's reluctance to treat mistakes about quality, value, or legal advantage as fundamental. It also protects settlements and compromises: parties frequently contract under uncertainty, and the later discovery that one side had a better position than expected does not necessarily undo the agreement.

Associated Japanese Bank is an important first-instance decision because Steyn J gave a structured analysis of common mistake. The bank entered a guarantee-like arrangement concerning machines that did not exist. Unlike Bell, the non-existence of the machines went to the foundation of the transaction. The case is often used pedagogically because it articulates the inquiry into the assumed state of affairs, fundamentality, impossibility, and risk. It anticipated much of the later formulation in The Great Peace.

The Great Peace is the modern centre of gravity. A vessel was hired to assist a distressed ship on the mistaken assumption that it was much closer than it was. The Court of Appeal held that the contract was not void: performance was still possible, and the mistake did not make the service essentially different. The court also rejected the separate equitable doctrine associated with Solle v Butcher. This is the case to cite for the contemporary common law test and for the proposition that equity does not provide a looser escape route.

Raffles v Wichelhaus supplies the classic mutual mistake example. Where contractual language points equally to two different things and neither meaning can objectively be preferred, there may be no contract. Modern exam use should be cautious: many apparent Raffles problems are solved by construction. Only where interpretation fails should the answer move to no contract for mutual mistake or uncertainty.

Cundy v Lindsay and Shogun Finance form the main identity line. Cundy held that the original seller did not contract with the rogue who impersonated a reputable firm by correspondence. Shogun held, by majority, that a finance company contracted only with the named customer in the written hire-purchase documents, so the rogue acquired no title. These cases show the severity of voidness and the importance of distinguishing identity from mere attributes. They also show that mistake is not only about formation doctrine but about loss distribution between innocent parties.

See practice questions section below.

See further reading section below.