This is “Mistake”, section 10.3 from the book The Law, Sales, and Marketing (v. 1.0). For details on it (including licensing), click here.

For more information on the source of this book, or why it is available for free, please see the project's home page. You can browse or download additional books there. To download a .zip file containing this book to use offline, simply click here.

Has this book helped you? Consider passing it on:
Creative Commons supports free culture from music to education. Their licenses helped make this book available to you. helps people like you help teachers fund their classroom projects, from art supplies to books to calculators.

10.3 Mistake

Learning Objectives

  1. Recognize under what circumstances a person may be relieved of a unilateral mistake.
  2. Recognize when a mutual mistake will be grounds for relief, and the types of mutual mistakes.

In discussing fraud, we have considered the ways in which trickery by the other party makes a contract void or voidable. We now examine the ways in which the parties might “trick” themselves by making assumptions that lead them mistakenly to believe that they have agreed to something they have not. A mistake is “a belief about a fact that is not in accord with the truth.”Restatement (Second) of Contracts, Section 151.

Mistake by One Party

Unilateral Mistake

Where one party makes a mistake, it is a unilateral mistakeA mistake made by one party to a contract; relief is not usually granted.. The rule: ordinarily, a contract is not voidable because one party has made a mistake about the subject matter (e.g., the truck is not powerful enough to haul the trailer; the dress doesn’t fit).


If one side knows or should know that the other has made a mistake, he or she may not take advantage of it. A person who makes the mistake of not reading a written document will usually get no relief, nor will relief be afforded to one whose mistake is caused by negligence (a contractor forgets to add in the cost of insulation) unless the negligent party would suffer unconscionable hardship if the mistake were not corrected. Courts will allow the correction of drafting errors in a contract (“reformation”) in order to make the contract reflect the parties’ intention.Sikora v. Vanderploeg, 212 S.W.3d 277 (Tenn. Ct. App. 2006).

Mutual Mistake

In the case of mutual mistakeErroneous belief shared and relied on by both parties to a contract for which a court often grants relief.—both parties are wrong about the subject of the contract—relief may be granted.

The Restatement sets out three requirements for successfully arguing mutual mistake.Restatement (Second) of Contracts, Section 152. The party seeking to avoid the contract must prove that

  1. the mistake relates to a “basic assumption on which the contract was made,”
  2. the mistake has a material effect on the agreed exchange of performances,
  3. the party seeking relief does not bear the risk of the mistake.

Basic assumption is probably clear enough. In the famous “cow case,” the defendant sold the plaintiff a cow—Rose of Abalone—believed by both to be barren and thus of less value than a fertile cow (a promising young dairy cow in 2010 might sell for $1,800).Sherwood v. Walker, 33 N.W. 919 (1887). Just before the plaintiff was to take Rose from the defendant’s barn, the defendant discovered she was “large with calf”; he refused to go on with the contract. The court held this was a mutual mistake of fact—“a barren cow is substantially a different creature than a breeding one”—and ruled for the defendant. That she was infertile was “a basic assumption,” but—for example—that hay would be readily available to feed her inexpensively was not, and had hay been expensive, that would not have vitiated the contract.

Material Effect on the Agreed-to Exchange of Performance

“Material effect on the agreed-to exchange of performance” means that because of the mutual mistake, there is a significant difference between the value the parties thought they were exchanging compared with what they would exchange if the contract were performed, given the standing facts. Again, in the cow case, had the seller been required to go through with the deal, he would have given up a great deal more than he anticipated, and the buyer would have received an unagreed-to windfall.

Party Seeking Relief Does Not Bear the Risk of the Mistake

Assume a weekend browser sees a painting sitting on the floor of an antique shop. The owner says, “That old thing? You can have it for $100.” The browser takes it home, dusts it off, and hangs it on the wall. A year later a visitor, an expert in art history, recognizes the hanging as a famous lost El Greco worth $1 million. The story is headlined; the antique dealer is chagrined and claims the contract for sale should be voided because both parties mistakenly thought they were dickering over an “old, worthless” painting. The contract is valid. The owner is said to bear the risk of mistake because he contracted with conscious awareness of his ignorance: he knew he didn’t know what the painting’s possible value might be, but he didn’t feel it worthwhile to have it appraised. He gambled it wasn’t worth much, and lost.

Key Takeaway

A mistake may be unilateral, in which case no relief will be granted unless the other side knows of the mistake and takes advantage of it. A mistake may be mutual, in which case relief may be granted if it is about a basic assumption on which the contract was made, if the mistake has a material effect on the agreed-to exchange, and if the person adversely affected did not bear the risk of the mistake.


  1. Why is relief usually not granted for unilateral mistakes? When is relief granted for them?
  2. If there is a mutual mistake, what does the party seeking relief have to show to avoid the contract?