Mistake in contract law represents a critical doctrinal boundary in Indian law between consent-based validity and objective contractual truth. A bilateral mistake—where both parties are mistaken about the same fundamental fact existing at the time of contract formation—renders the contract void ab initio because the essential agreement itself never materialised. This doctrine flows from the foundational principle that a contract requires genuine consensus ad idem, or meeting of minds on the same subject matter in the same sense. Under Indian contract law, when both parties operate under a shared misapprehension about a basic fact—such as the existence of goods being sold, the identity of the contracting party, or a fundamental quality that defines the contract's very purpose—the law refuses to enforce it because consent was illusory. The statutory framework treats such contracts as void from inception, with neither party able to seek specific performance or expect damages. Conversely, a unilateral mistake, where only one party misunderstands a material fact, generally does not vitiate the contract. This asymmetry exists because the law prioritises the protection of the innocent party's reasonable expectations and contractual security. If one party's internal misunderstanding could unwind agreements, commercial certainty would collapse; therefore, the mistaken party bears the consequence of their own error unless the non-mistaken party induced the mistake through fraud or misrepresentation, which operates under different legal principles. The distinction hinges on mutuality: bilateral mistakes destroy mutual assent; unilateral mistakes affect only one party's mental state, which the law treats as insufficient to overthrow an objectively concluded bargain. Understanding this principle requires grasping that contract law examines objective manifestations of intent, not subjective mental states, except where both parties share identical misapprehension about the factual substratum. The practical import is substantial: in a bilateral mistake scenario, both parties stand equally discharged; neither can sue for breach; remedies focus on restitution of benefits conferred. In unilateral mistake, the mistaken party typically remains bound unless they can prove fraud or unconscionable exploitation. Courts also recognise that certain mistakes qualify as 'mistakes of law' rather than 'mistakes of fact,' and law mistakes generally offer no relief because parties are presumed to know the law. The boundary between fundamental and non-fundamental facts is itself fluid and context-dependent. A mistake about whether goods have arrived, when both parties assume they have, differs fundamentally from a mistake about the precise market price of goods that form the contract's subject matter. Examiners often blur this distinction, presenting scenarios where the mistake seems material but falls outside the 'fundamental fact' requirement. Furthermore, the doctrine interacts complexly with related concepts like fraud, misrepresentation, and disclaimer clauses. A party cannot escape a unilateral mistake by claiming they relied on the other's silence if the contract terms explicitly exclude such reliance. The law thus balances the competing values of consent-based justice against contractual certainty and the reasonable expectations of the non-mistaken party. For CLAT aspirants, this principle is frequently tested through hybrids that blend mistake with fraud or conditional contracts, creating deliberate ambiguity about whether the mistake was truly bilateral or whether it resulted from one party's active concealment.