Thousands of people in the UK were charged twice for debit card payments.
The error occurred due to an issue with a particular card terminal run by Cardnet, a joint venture between Lloyds Bank and First Data.
Money came out of bank accounts twice but transactions only showed up once on many shop, pub and restaurant receipts.
Lloyds Banking Group said people would be refunded and just under 5% of Cardnet machine transactions were affected during part of 29 August.
Many people banking with different organisations were charged twice for spending money in shops, restaurants and businesses using Cardnet terminals as their way of taking payment.
The majority of affected transactions involved Visa debit cards.
Simon Alton, 39, discovered he had paid twice for a meal at Côte Brasserie, in Shrewsbury, Shropshire, last Wednesday.
“It’s a reminder to check your statement, it’s an unexpected thing to happen,” he said.
“I’m a bit lax, if it wasn’t for the friends I had been out with who messaged me the next day, I might not have noticed.”
Mr Alton, who works in PR, said he had spent about £60, adding: “The restaurant’s been very good. As far as we know, we’ll get the money back.”
Côte Brasserie has been approached for comment.
Other customers took to social media to express their concern about duplicate payments.
One woman, known as Lisa D, said she had visited the Oblix restaurant in The Shard, London, to celebrate her husband’s birthday when she was charged twice.
Sara McCluskey went to Solita NQ in Manchester with friend Oriel Hardman when both were affected.
Billy Hatch visited German Donner Kebab (GDK) in Sutton, Greater London, and had the same problem.
Other businesses responded by saying it was a problem with the card processing data firm and not an issue with individual businesses or banks.
Brunning Host Pubs, which runs pubs in Devon, released a statement to customers on 1 September after being made aware of the problem.
The Nab’s Head, in Preston, Lancashire, said in its statement second duplicate payments would not be taken by them but refunded as soon as possible.
Cardnet handles more than 1.2 billion transactions every year, according to its website.
It claims to make it “easy for merchants to accept card transactions online, in person or by phone”.
Simon Gompertz, BBC Personal Finance Correspondent
We are all using cards more to make payments.
Last year cards topped cash for the first time ever, as contactless paying took off.
So it’s vital we can have confidence in the security and reliability of the technology.
It has taken Cardnet nearly a week to send out refunds for its double-charging mistake.
In that time some customers will have been perplexed or even panicky about what happened to their accounts.
If banks want us to embrace the future, they need to be much sharper at dealing with the glitches.
A Lloyds Banking Group spokesperson said: “A small number of transactions processed last Thursday by Cardnet were duplicated.
“Action has been taken to rectify this situation as quickly as possible and we apologise for any inconvenience caused.”
In July one of the UK’s money transferring services had to deal with a backlog of payments after a system failure.
Visa experienced a similar issue in June that left customers across Europe unable to make some purchases.
While in May, TSB customers continued to face issues with online banking.