Ombudsman warns there is no such thing as free cash
Bank customers have been warned that they are unlikely to be allowed to keep money mistakenly credited to their bank account.
About 100 cases of so-called "mis-applied credit" are dealt with by the Financial Ombudsman Service each year.
Only in a few rulings has the ombudsman allowed bank customers to keep the cash.
The ombudsman received 39,576 new complaints about all financial services in the first three months of 2010.
The free ombudsman service was set up by law to settle complaints between consumers and financial businesses on anything from pawnbroking to mortgages.
Mis-applied credit is counted under complaints about current accounts made to the ombudsman.
There were 24,515 complaints about current accounts during the 2009-10 financial year. There have been an additional 5,420 in the first three months of this financial year.
Some people complain that after a bank has mistakenly added money into their account, the bank then chases them for the return of the cash.
Although the customer might argue that the error was the fault of the bank and so the customer should keep the money, the ombudsman usually favours the bank.
"In the board game Monopoly, it is good news if you get a card telling you that the bank has made an error in your favour - as you do get to keep the money," said chief Financial Ombudsman Natalie Ceeney in Ombudsman News.
"But in real life, things are different.
"When dealing with complaints about mis-applied credit, we generally take the view that consumers are required to return any money paid to them by mistake."
An example was when one customer had £1,000 transferred into his account. The bank accidentally duplicated the transfer, putting another £1,000 in the account.
The bank asked for the extra £1,000 to be repaid two weeks later, but the customer refused, saying that it was the bank's mistake and that he had already spent the money.
The ombudsman ruled that he should pay the money back to the bank, which was acting reasonably when asking for the money in 10 interest-free instalments.
However, in a separate case, the ombudsman decided that a customer could keep the extra cash.
Her account had been credited with £1,000 after she paid in a cheque of just £100. After spotting the error when checking a balance on a cash machine, the customer spoke to a cashier inside a branch, who confirmed it was in fact correct.
The recently-retired customer was expecting a tax rebate and assumed the money was from this. She took her niece on a holiday to Italy with the money.
Nearly four months later, she was "distressed" to receive a demand for £900 from the bank.
The ombudsman ruled that she had acted in good faith, had checked with the cashier and could not recover any of the money spent on the holiday.
As a result, the ombudsman ruled that the bank was not entitled to demand the £900 back.
The Financial Ombudsman has published quarterly data for the first time which shows that inquiries about payment protection insurance (PPI) again top the list of complaints.
PPI should enable borrowers to pay off loans such as credit cards or mortgages if they fall ill or lose their job, but sales have come under fire.
There were 13,520 complaints in the first three months of the financial year, 81% of which were resolved in favour of the consumer.
This was more than twice the number of complaints about current accounts (5,420) which was next on the list, followed by queries about credit card accounts (4,296).
The number of consumers inquiring about travel insurance complaints increased by 28% in the first six months of this year, the ombudsman added.