Also known as:mutual mistakes of fact and law · mutual mistake · mistake of fact and law
Written by attorneys · grounded in primary & secondary sources — see below
A contract defense that renders an agreement voidable when both parties share an erroneous belief about an existing fact or legal status that was a basic assumption of the bargain and materially affects the exchange, unless the adversely affected party bears the risk of the mistake.
Sources & Authorities
How it applies
Common Examples
2
Zoning Error After Deed Delivery
Mariam Mansour contracted to buy an apartment building from Matrix Technologies for a price based on short-term rental income. Both parties believed existing zoning permitted those rentals. After closing and deed acceptance, they discovered a recent overlooked amendment that banned the use and slashed the property value. Mariam seeks to avoid the deal. The mutual mistake about the legal status of the property use allows rescission despite merger of the contract into the deed.
Collateral Promise Surviving Merger
Melissa Mills bought land from Monarch Pharmaceuticals under a contract that included an oral promise to build a boundary fence after closing. The deed contained no reference to the fence. After accepting the deed, Melissa discovered both parties had shared the mistaken belief that the fence obligation would remain enforceable. The mutual mistake permits her to enforce the promise even though merger would otherwise discharge it.
Put it into practice
Test Yourself
9
Practice Questions5
· 9 primary sources
Select any source to read its text and confirm it supports the definition.
Cases
Common Law
Restatements
Casebooks
Common questions
Frequently Asked
3
When does a mutual mistake about zoning permit a buyer to avoid a real estate contract?+
A contract is voidable when both parties are mistaken about a basic assumption that materially affects the exchange and the adversely affected party does not bear the risk. Both parties here believed the zoning allowed short-term rentals that justified the price. Discovery of the prohibition shows the assumption was false and the mistake has a material effect on value. No facts indicate the buyer assumed that risk, so avoidance is available.
Supporting sources
Does mutual mistake about the authenticity of artwork allow rescission after payment and delivery?+
Yes when both parties believed the work was an original by a famous artist and that belief was a basic assumption driving the price. Scientific testing later revealed a forgery of minimal value. The error has a material effect on the exchange. Nothing shows the buyer knowingly accepted uncertain provenance, so the contract is voidable.
Supporting sources
How does merger interact with a claim based on mutual mistake in a real estate transaction?+
Merger discharges contractual promises not embodied in the deed, but does not bar claims based on mutual mistake. A buyer who accepts the deed may still avoid or reform the transaction when both parties shared an erroneous belief about a basic fact or legal status that materially affected the deal.
Supporting sources
ContractsDefenses to enforceability · Mistake and misunderstandingNEXTGENFoundational