A civil-law concept defining a rule, doctrine, or procedural mechanism used to resolve private disputes. It applies when its legally specified conditions are satisfied and produces defined consequences in adjudication or settlement. It does not apply where required elements or prerequisites are not met. It materially affects rights, obligations, or available remedies in civil controversies. The concept is generally stable, though statutory and doctrinal refinements may occur over time.

Principle

Where both parties operate under the same material mistake about an existing fact or identity and neither party assumed the risk of that mistake, the law may permit rescission or reformation to restore the parties to their pre-contract position.

Demonstration

Buyer and seller contract for a named painting believed by both to be an authentic work by a named artist; after discovery that both were mistaken and the painting is a copy, the mistake is mutual and fundamental, permitting rescission.

Misapplication

Treating ordinary bargaining errors, differences in price expectation, or disputes over subjective quality as mutual mistake; or attempting to avoid a contract when a party expressly allocated the risk of the relevant fact.

Consequence

When mutual mistake is established the contract may be rescinded with restitution, or reformed if feasible to reflect the parties’ true agreement; parties may be returned to status quo ante rather than awarded expectation damages.

Reversal

Contrast with unilateral mistake (one party mistaken) and misrepresentation (false statement inducing assent); where only one party erred, different remedial rules apply.

Boundary

Requires a mistake as to a present, material fact or identity; excludes mistakes about future events, mere regret, or differences in interpretation where agreement on the essential fact existed.

Semantic Tension

Confused with mutual assent failures, mutual misunderstanding, or gap-filling doctrines; tension centers on whether the mistake negates a meeting of the minds or merely affects value.

Synthesis

Mutual mistake is a bilateral error doctrine: when both parties share a material, existing factual error that destroys the common basis of their bargain and neither bore the risk, the contract may be undone or adjusted to reflect what the parties actually intended.