This article examines a recurring issue in finance disputes: the demand for payment or service of an acceleration notice in circumstances where the borrower contends that the relevant contractual precondition has not been satisfied. An invalid acceleration notice is generally characterised as a nullity rather than a breach of contract. Where a borrower pays for commercial reasons even though it believes that the sum is not due, restitution is generally unavailable because the borrower cannot show that it paid under a mistake.