The effect of mistake upon a bargain may be various: 1. A mistake may be such as to prevent any real expression of agreement, in which case the transaction is void. 2. There may be an expression of agreement which differs from the agreement intended by the parties. Here it may be equitable to reform the expression in conformity with the intention or, if not, to rescind the transaction. 3. There may be an expression of agreement and the expression may conform to the intention of the parties, but a mistake as to the way the agreement would apply to existing facts may make it equitable to rescind the transaction. 4. The mistake may be wholly without legal consequences.

15 E. g., Cal. Civ. Code, Sec. 1577. So 2 Pomeroy Eq. Jur., Sec. 839.

In an earlier part of the book the question has been considered how far mistake prevents the formation of a contract; 16 and the fourth class will be sufficiently indicated by assigning to it all cases not falling in the second or third class. In these two classes the parties have made contracts; the questions to be dealt with concern their right to avoid them altogether or to substitute others in their place.