mutual material mistake

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In contract law, a defense used by one party to argue that a contract is invalid. In order to use the defense of mutual material mistake to argue that formation of the contract was improper, a party must argue that: (1) there was a mistake; (2) that the mistake must be material, meaning, that it must concern substantive characteristics of the subject of the contract; and (3) the mistake was mutual, meaning both parties had the same mistaken belief.