Mistake
Definition
An erroneous belief about something material to the contract. A bilateral mistake of fact (both parties mistaken) renders a contract Void. A unilateral mistake (only one party) generally does not affect the validity of the contract.
Example
"A and B both believe that a painting being sold is an original masterpiece, but it is actually a copy. Both parties are mistaken — the contract is Void due to bilateral mistake of fact."