The making of mistakes by organisms and other living systems is a theoretically and empirically unifying feature of biological investigation. Mistake theory is a rigorous and experimentally productive way of understanding this widespread phenomenon. It does, however, run up against the long-standing ‘functions’ debate in philosophy of biology. Against the objection that mistakes are just a kind of malfunction, and that without a position on functions there can be no theory of mistakes, we reply that this is to misunderstand the theory. In this paper we set out the basic concepts of mistake theory and then argue that mistakes are a distinctive phenomenon in their own right, not just a kind of malfunction.