Titles in the Core Text series take the reader straight to the heart of the subject, providing focused, concise, and reliable guides for students at all levels. This chapter examines the situation where both parties to a contract share a common mistake. It analyses several court cases indicating that certain sorts of mistake can render contracts void at the level of common law. It discusses the orthodox approach which asserts that there is a separate legal doctrine whereby certain sorts of common mistakes inevitably render a contract void; it also considers an alternative way of conceptualising common mistake cases, the construction approach, which argues that the effect of common mistake is ascertained by construing and interpreting the contract. This chapter also considers the scope of the equitable remedy of rectification for common and unilateral mistake, which gives the court the jurisdiction, in exceptional cases, to correct transcription mistakes in the parties’ written contractual document.