Boxtops is right, someone has probably reversed a couple of the numbers and the debit has gone to someone elses account. If you go in and advise them they will probably leave the interest accrued. However if it does not come to light until the other party need some money or get their statement, the bank will have to recredit them while tracing the funds. They will also know you were aware from your statement dates, so will take steps to cancel the extra interest, and it may endanger your relationship with the bank for the future.