On 11 August 2020, Citibank made a payment of US$900m to the lenders under a syndicated loan agreement, in what the US court later described as “one of the biggest blunders in banking history”. When Citibank sued for the return of the money, the US court held that it was not entitled to repayment because the payments discharged an existing debt and the recipients had no notice of the mistake. That decision, on 16 February 2021, has understandably caused consternation among bankers. One report described the decision as “eye-watering” and one that “will strike terror into earnest hearts in the global trust and agency community”. London, of course, is another global financial hub and the industry might be equally interested to know how this dispute might have been determined as a matter of English law. This article addresses that question.