8. Mistake

Poole’s Casebook on Contract Law provides a comprehensive selection of case law that addresses all aspects of the subject encountered on undergraduate courses. This chapter considers the area of ‘mistake’. The law distinguishes between several types of mistake. Some mistakes (‘agreement mistakes’) prevent formation of an agreement. These mistakes are mutual mistakes (where the parties are at cross purposes) and unilateral mistakes (where one party is mistaken and the other knows or ought to know this, e.g. unilateral mistake as to identity). The chapter also looks at document mistakes and specifically rectification of a written document to reflect accurately what the parties in fact agreed, and the plea of non est factum (‘this is not my deed’). Finally, a contract having no contractual allocation of risk and made under the same mistaken assumption may be void for ‘common mistake’ if the mistake is so fundamental that it ‘nullifies’ consent. This is known as ‘initial impossibility’ because the impossibility already exists when the parties agree to the contract. This chapter deals with common mistake and initial impossibility, contractual risk allocation, and the theoretical basis for the doctrine of common mistake. It discusses categories of fundamental common mistake, including res extincta, and assesses the legal effects of mistakes as to quality made by both parties. The chapter concludes by considering the relationship between common mistake and frustration.