The building society has been forced to apologise to thousands of customers after a technical glitch in Northern Ireland led to accounts not being debited at the time of withdrawals.
The problem reportedly affects about 7,500 customers who took money from their accounts using cash machines at Northern Bank between last November and this February.
The total amount in question is said to be about £400,000. An IT fault in the building society's connection to the national Link processing system is believed to be the cause of the problem.
Jay Colville, area director of Nationwide in Northern Ireland, told the BBC that Link upgraded the ATM network at the end of last year, but that the building society did not handle the upgrade correctly.
Nationwide has told affected customers it will now debit the cash from their accounts on 10 March.