The consequences here are stark: an operative mistake renders a contract void from the outset, defeating even an innocent third party's title, so the doctrine is kept deliberately narrow. This chapter begins with the foundations — what makes a mistake operative, the main categories, and the void-voidable distinction on which everything turns. It then works through the categories themselves: common mistake and res extincta, mutual mistake and the objective test that resolves it, unilateral mistake as to identity with its face-to-face presumption, and unilateral mistake as to terms. It closes with the plea of non est factum, the equitable remedy of rectification for documents that misrecord the parties' shared intention, and how a third-party purchaser can take good title under a contract voidable for fraud.
