Garth owns two all-terrain vehicles (ATVs), worth $1,000 and $500, respectively. Helen agrees to buy "Garth's ATV" for $750. Garth believes, in good faith, that he is selling the $500 ATV. Helen believes, in good faith, that she is buying the $1,000 ATV. In this situation

There is no contract since both Helen and Garth made a mutual mistake.

In contract law, a mutual mistake occurs when all the parties involved (Helen and Garth) are mistaken about important material facts that affect the contract (which ATV is being sold). The parties intend to perform but what they consider being part of the contract is not what the other party considers part of the contract. When both parties make a mutual mistake, the contract is cancelled.

Mutual mistakes are not on purpose, they are mistakes committed in good faith.