SDCL § 53-4-9: Mistake of fact defined.

Mistake of fact is a mistake not caused by the neglect of a legal duty on the part of the person making the mistake and consisting in:

(1) An unconscious ignorance or forgetfulness of a fact, past or present, material to the contract; or

(2) Belief in the present existence of a thing material to the contract which does not exist, or in the past existence of such a thing which has not existed.