Contract

If one party has made a mistake as to the terms of the contract and that mistake is known to the other party, then the contract is not binding. However, if one party has made a mistake about a fact on which he bases his decision to enter into the contract, but that fact does not form a term of the contract itself, then, even if the other party knows that the first is mistaken as to this fact, the contract will be binding.

Thus, the rule that a unilateral mistake means a prima facie agreement is not binding only applies when there is a unilateral mistake as to a contract term. There is no equitable jurisdiction to grant rescission of a contract where one party has made a unilateral mistake as to a fact or state of affairs which is the basis upon which the terms of the contract are agreed, but that assumption does not become a term of the contract.