Under Section 20 of Indian Contract Act, agreements caused by bilateral mistake of fact are VOID (not voidable). If both parties mistake something fundamental (like subject matter existing), the agreement is void ab initio - never valid. Contracts caused by fraud (Sec. 19), misrepresentation (Sec. 19), or coercion (Sec. 19) are VOIDABLE (valid until aggrieved party chooses to avoid). Only bilateral mistake makes the contract itself void, not merely voidable. This is a key distinction.