Under Section 20 of Indian Contract Act, agreements where both parties are under a mistake as to a matter of fact essential to the agreement are void. Bilateral mistake about a fundamental fact (like existence of subject matter) means no true consent was given - there's no meeting of minds. Unilateral mistake (one party mistaken) generally doesn't void a contract. Fraud/misrepresentation make contracts voidable (not void). Contingent agreements are valid.