Thousands of Nationwide's overdraft customers are being refunded £6m after the building society failed to send them the correct text warnings.
Bank customers must receive a text alert before they are charged for entering unarranged overdrafts.
A total of 320,000 customers were affected and are now in line for a reimbursement of the charges.
The refunds come as Nationwide deals with a separate technical error, when payments appear to be taken twice.
Customers have expressed anger on social media over the errors, which meant account balances looked too low. Thousands have been affected, and the building society has apologised.
Why will Nationwide pay refunds?
On a bad day for the building society, the Competition and Markets Authority (CMA) announced that Nationwide had broken rules over text alerts 20 times since February last year.
The warnings are supposed to give customers time to take action to avoid unexpected overdraft charges. In the majority of cases, Nationwide failed to make it clear in texts that customers could incur charges if they did not take any action.
"Nationwide failed to do this on numerous occasions and our action today makes it clear they must fix this as a matter of urgency. It is imperative that these problems are sorted out immediately and that they don't occur again," said Adam Land from the CMA.
The building society apologised and said the wording was corrected in November 2018, while additional controls would ensure this did not happen again.
The CMA does not have the authority to fine companies which breach the rules, although it is calling on the government to bolster its powers.
What is the fresh technical fault?
Some angry customers have been threatening to leave Nationwide over a fault that suggested they had less money in their accounts than should have been the case.
The error meant some customers appeared to have made payments twice, with one completed, and a duplicate marked as a pending transaction.
One customer believed they had just £2.98 left in their account, while another appeared to be overdrawn even though he did not have an overdraft facility.
Another customer, Iwan Edwards, said he was charged twice for a £12,543 payment for a new car, and did not know what was going on for 12 hours.
Clare Jones, from Kidderminster, was paying £2,000 towards a holiday, but the sum appeared to have been taken out twice.
"I went to buy something online, but it said my bank wouldn't authorise payment. I looked online and it said we were about £2,000 overdrawn," she said.
"They said there was nothing they could do, they left us with no money."
A Nationwide spokeswoman said: "Yesterday evening, we identified a problem that meant the available balance on some member accounts appeared lower than they should, due to some payments still showing as pending.
"This was corrected overnight and everything is now working normally. We apologise to those members affected and can assure them that they won't incur any charges as a result of this issue."