This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
It is often said that mistake' must be mutual, which seems to mean that both parties must make the same mistake, or one must make it and the other must know and take advantage of it. Like many other general propositions concerning mistake, this does not prove, a satisfactory guide on analysis. Of the mistakes as to an essential feature of the contract, mistake as to the identity of the adversary party can scarcely be conceived of as mutual. As to mistakes concerning the subject-matter, courts differ; some holding that such mistakes must be mutual to be operative;1 others that they need not be.2 Mistake as to the terms of a contract can not be mutual, yet it often is operative.3 Mistake in the inducement is not usually operative; hence, the necessity of its being mutual need not be considered.4 Mistake in expression, to be operative as such must be mutual.5 The term "mutual" in this sense has a peculiar meaning.6 Doubtless the rule originated in this class of mistake, and its application to other classes is partly due to a loose and undiscriminating statement of such rule as a general proposition. Payment by mistake of fact has the same effect whether the mistake is mutual or not.7 to Leonard v. Howard, 67 Or. 203, 135 Ac. 549.
11 Steinmeyer v. Schroeppel, 226 111. 9, 10 L. R. A. (N.S.) 114, 80 N. E. 564.
12 Bevins v. Coates (Ky.), 96 S. W. 5S5.
1 Thus if A is buying land from B and X points out the wrong land and A makes no further inquiry A's right to take advantage of such mistake depends on whether X is B's agent; in which case A can avoid the contract; or is not, in which case A can not:
McKinnon v. Vollmar, 75 Wis. 82, 17 Am. St. Rep. 178, 6 L. R. A. 121, 43 N. W. 800; (obiter) Law v. Grant, 37 Wis. 548.
2 Strong v. Lane, 66 Minn. 94, 68 N. W. 765; Buck v. Equitable Life Assurance Society, 96 Wash. 683, 165 Ac. 878.
3 See Sec. 274.
4 See ch. XIII.
5 See ch. LXX.
6 See ch. LXX. 7 See Sec. 563.