Speed is of the essence for banks and their customers to retrieve funds when errors are made during money transfers, an ombudsman has said.
The Financial Ombudsman Service received 688 complaints in the last financial year about money transfers, up from 529 the previous year.
But fewer than half of these cases were concluded in the customers' favour.
The ombudsman said any transfer, online or in a branch, relied on account details being entered correctly.
"It is inevitable that problems will sometimes arise," said Natalie Ceeney, in the service's information publication Ombudsman News.
"We see a significant number of complaints involving money being transferred to an unknown third party by mistake.
"In some cases the money can be recalled, but this relies on the customer and the financial business acting quickly."
The ombudsman said that mistakes typically occurred because of payment details being entered incorrectly.
If this happens, the responsibility is not always with the bank. This was shown in one example, when a father transferred £1,000 to his daughter who was at university.
A week later, she had not received the funds. It emerged that one of the digits of the account number that he entered had been wrong.
He complained, saying that the bank should have checked that the name and account number matched, and that the fact it had not done so had caused him significant inconvenience.
However, the ombudsman sided with the bank, saying that it had displayed a clear message on the screen at the start of the online payment process, explaining that only a sort code and an account number would be used to process a payment - and not an account name.
Yet, in another case, the ombudsman ordered that a bank customer be refunded £150 and awarded compensation of £75.
In this case, the customer went online to transfer £150 to his mother's bank account. As he had sent money to her before, he was able to click on her details from a list of recent transactions.
However, he clicked the wrong recipient details and the money was transferred to someone from whom he had bought a secondhand computer.
The ombudsman ruled that the bank had not given him sufficient opportunity to check the transaction details. In particular, it noted that there was no "final confirmation" page.
Other issues seen by the ombudsman included delays and administration problems, disputes over currency rates, and arguments over fees and charges for sending money.