Mistake of both the parties about subject matter renders an agreement

A bilateral mistake (both parties mistaken) about a fundamental fact going to the root of the agreement makes it void ab initio under Section 20 of the Indian Contract Act. The mistake must be about a fact essential to the agreement, not just about terms or quality. This differs from unilateral mistake which may only make an agreement voidable. A mutual mistake about the subject matter itself destroys consensus ad idem.