A TSB customer was given access to someone else's £35,000 savings account, £11,000 Isa, and a business account when he logged onto his account last night.
Matthew Neal had simply wanted to check how much he'd spent at the pub the night before.
He's just one of potentially millions of TSB customers affected by a massive computer failure.
The bank's upgrade to its banking systems has caused chaos for customers.
The bank warned its users that it was carrying out upgrades at the weekend between 16:00 BST on Friday and 18:00 on Sunday.
But a number of customers reported they were still facing problems on Monday morning, with many worried about being able to pay bills or even get cash to pay for essentials.
TSB apologised and claimed it had fixed the issue. In a statement on Monday it said: "We're really sorry that some of our customers experienced problems accessing our mobile app and internet banking yesterday evening. Both of these services are now up and running again."
Many customers took to Twitter to complain of being unable to access their accounts despite the bank's claim. The bank's social media staff simply repeated the line: "Unfortunately, there are some intermittent problems affecting this service so please bear with us. We're working as hard as we can to resolve this."
'I had access to a £35,000 savings account, £11,000 ISA and a business account'
Matthew Neal from Hertfordshire checked his TSB app after 6pm on Sunday evening to find out his balance and see how much he had spent at the pub the night before. But he got a shock.
"I could see all my accounts, but on top of that also three accounts belonging to someone else: a £35,000 savings account, an £11,000 Isa and a business account," he told the BBC.
"I could see their account numbers, sort codes and transaction histories and I had access to transfer money too, if I was that way inclined.
"The thing that was worrying me most was: what if someone can see mine too?"
He tried to ring the bank to alert it to the problem but after 45 minutes on hold, gave up.
Since then he has been unable to access his accounts at all.
A TSB spokesperson commented: "We have noted some reports in the media regarding customers' access to account information last night. We can confirm the access issues, which lasted only about 20 minutes and impacted just a tiny fraction of our customer base, were fixed last night."
What caused the problems?
When TSB split from the Lloyds Banking Group, it continued to rent a banking platform from its former owner while it developed its own "state-of-the-art" platform.
That was ready to go live at the weekend and TSB moved its customers' data from the Lloyds' platform to its own. This was a long-planned disruption to the service and the bank said it informed customers of the change and that it would lead to them being unable to use online banking or payment systems at the weekend.
We’re planning an upgrade to our systems between 4pm Friday 20th of April until 6pm Sunday 22nd of April. We’re sorry but during this time some services, like online banking, making payments or transferring money won’t be possible. For details please visit https://t.co/jSmnTgJcDZ pic.twitter.com/H3JWJtgNxC— TSB (@TSB) April 19, 2018
That led to two problems. First, many customers said they were unaware of the changes and so were caught out. Second, customers experienced difficulties long after the 6pm deadline on Sunday evening that TSB promised that things would be sorted by. In fact many were still complaining of being unable to access services on Monday morning.
My balance showed £13,000 despite me being in the red
Laim McKenzie, from Paisley, in Scotland, signed into his app shortly after 6pm on Sunday evening to discover the problem.
The 20-year-old said: "I saw the details of one other account holder, as well as my own.
"My balance, because of my overdraft, is in minus, but my balance was showing at £13,000.
"I closed the app and went in again at 19:00 and only my account was showing - but my balance was showing at zero.
"I've closed the app again and have not been able to access it since."
Social media storm as angry customers take to Twitter
Many customers took to Twitter to complain about issues, including being unable to log into their internet banking accounts, being unable to transfer funds and not being able to make payments.
@TSB_News why is the upgrade still not over? My online banking is still not working— tara jackson (@tarajackson__) April 23, 2018
Still can’t do any kind of banking/accounts admin as @TSB still in a mess. I left NatWest when they lost some cash I never got back a few years ago. Time to jump ship to another bank again?— Zoe South (@EdibleDormouse) April 23, 2018
Why is nothing every delivered well in this world??!! App doesnt work just says 'something went wrong' and website logon is like wading through treacle.... why bother upgrading? not really surprised though— Nigel Catt (@CattClan) April 22, 2018
hi @TSB neither app or desktop site are working for me - keep getting error messages. When will these be fixed?— Lisa S (@IsItJustMeme) April 22, 2018
TSB has issued a statement apologising to customers for the problems.
A spokeswoman said: "Unfortunately, there are some intermittent problems affecting these services.
"Both the app and internet banking should be up and running again soon."
The Financial Conduct Authority, which has the power to fine banks for system failures, said it was "aware of the issue" and in touch with TSB.
The Information Commissioner's Office also said it was "making enquiries" about a "potential data breach".