Staff at a branch of Barclays left the bank completely unlocked on Saturday and were only alerted to the problem after customers raised the alarm.
Anyone who chose to wander into the branch in Leigh-on-Sea in Essex had complete access to the site, from the cashiers' area to the vault, customers have said.
"It was astounding," Lesley Phillips told the Daily Mail. "There wasn't a soul in sight, and the whole place was open."
Phillips, along with two others, raised the alarm at around 2pm.
"There simply wasn't anybody there. We didn't know what to do, so we decided to phone the police and stand guard until they arrived," she said.
Police arrived at the bank by 3pm, with Barclays' security staff getting there at about 6pm.
Faulty lock to blame
Barclays thanked the honest customers for alerting them.
"We take these matters extremely seriously and have now carried out a full investigation and found the door locking mechanism was faulty," a spokeswoman from Barclays confirmed.
"We have taken steps to rectify this fault and would again thank the members of the public for bringing this matter to our attention.
"We confirm that no customers will be at risk as a result of this incident."
More common than you think
It's not the first time bank staff have managed to leave their branch open to all comers.
Last year a member of the public alerted the police about an unlocked front door of the HSCB branch in Tadley, Hampshire, while in 2008 a branch of NatWest was discovered to be unlocked in Dewsbury, West Yorkshire.
There have also been recent incidents in Newton Abbot, Devon, and Easingwold, North Yorkshire.