The document discusses different types of mistake in contracts, including common mistake, unilateral mistake, and mistake as to identity. For common mistake, a contract may be void if there is a mistaken belief about a fundamental fact like the existence or quality of the subject matter. For unilateral mistake, the contract may be void if one party is aware of the other's mistake regarding a term. Mistake as to identity looks at whether parties intended to contract with each other or someone else. The remedies are different depending on whether the mistake renders the contract void at law or voidable in equity.