Mistakes

Unilateral
Where 1 of the parties is mistaken to a material fact, the agreement is never-the-less binding unless the nonmistaken party knew of the error or enforcing the contract would be unconscionable.

Caveat emptor - buyer beware

Unilateral mistake of law

ignorance of the law is no excuse

Mutual mistakes -
where both parties are operating under the same material mistake of fact or law the contract is voidable