Legal Definitions - unilateral mistake

Definition: Unilateral mistake is a type of mistake in which only one party to a contract makes an error or has a misunderstanding about a fact or law. This mistake can make the contract voidable, but it is not as likely to do so as a mutual mistake.

Example: John wants to buy a car from Jane. Jane tells John that the car has never been in an accident, but in reality, it was in a major accident and was repaired. John believes Jane and buys the car. Later, John finds out about the accident and wants to return the car. This is a case of unilateral mistake because only John made the mistake of believing Jane's false statement.

This example illustrates how a unilateral mistake can affect a contract. John's mistaken belief about the car's accident history influenced his decision to buy the car. If John had known the truth, he might not have bought the car or would have negotiated a lower price. Because Jane made a false statement, John can argue that the contract is voidable due to his unilateral mistake.