The equitable doctrine of mistake as the basis for setting aside a voluntary disposition, such as a gift, is of much longer standing than the rule in Re Hastings-Bass, and is of much broader application. It is concerned essentially with the impairment of the disponor’s intention: he or she would not, for example, have made a gift, or settled property on trust, or exercised a power of appointment, or consented to the exercise of a power, but for some mistake. It is not limited in its application to trustees or fiduciaries, and indeed is most commonly seen in the context of individuals dealing with their own beneficially owned property. It has obvious affinities, though is not wholly aligned, with other instances of mistake in the law of obligations: most obviously common mistake as a ground for treating a contract as void, and causative mistake as an ‘unjust factor’ in the law of unjust enrichment. However, it also raises questions which are relevant to any analysis of the Re Hastings-Bass rule, not least since both principles may be engaged in the same factual scenario.