Nationwide, the UK's biggest building society, apologised today for a technical error that left 700,000 customers out of pocket.
Debit card transactions made on Tuesday 24 July were processed twice, affecting an estimated 704,426 accounts due to "human error".
"A small number of customers will have been adversely impacted," says Jenny Groves, divisional director for customer experience at Nationwide. "All charges will be refunded in full and any costs associated with this error will be reimbursed in full."
No financial loss
All debit payments were deducted as normal on Tuesday, but were then repeated on Wednesday, causing some customers to become overdrawn.
But Groves is adamant that "none of our customers will suffer financial loss as a result of this one-off error".
In another drama for British bank customers, NatWest's online banking service froze yesterday, leaving up to 11.5 million customers unable to log into their accounts or access their cash.
RBS, which owns NatWest, has stated that "online banking is now fully operational and debit card transactions are processing as normal. We continue to monitor the systems closely and will keep our customers fully informed. We apologise for any inconvenience caused".