What is the test for making a mistake in a contract?

As noted in Guthrie v. Times-Mirror Co. (1975) 51 Cal.App.3d 879, 885, 124 Cal.Rptr. 577, "[w]here parties are aware at the time the contract is entered into that a doubt exists in regard to a certain matter and contract on that assumption, the risk of the existence of the doubtful matter is assumed as an element of the bargain. [Citations.] Otherwise stated, the kind of mistake which renders a contract voidable does not include `mistakes as to matters which the contracting parties had in mind as possibilities and as to the existence of which they took the risk [citations].'"