Abstract
This chapter assesses situations in which one or both parties enter into a contract on the basis of a mistake that is so serious that it negates their consent to a contract; or, it means they did not consent to the agreement in the first place. Following such an ‘operative’ mistake, the contract will be void from the start and therefore treated as though a valid contract never existed. The chapter then considers the law on mistake. It starts with mistakes that prevent the formation of an agreement. The most significant mistake of this type is known as a ‘unilateral mistake’, which is where one party appears to have entered the contract on the basis of a mistake. The next significant issue is known as ‘common mistake’, which is where, at the time of creating the contract, both parties appear to be making the same mistake about the existence of an essential state of affairs. Finally, the chapter addresses the related remedy of equitable rectification before finally turning to the highly exceptional defence of non est factum.
