The machine was giving too much money for about six hours
Crowds queued at a cash machine in Manchester which gave double the amount of cash requested.
Word spread quickly about the error at the machine at the BP garage on Barton Road in Stretford on Thursday.
The giveaway went on for about six hours before the fault was reported to Nationwide building society.
The firm said it was "disappointing" that no-one reported it earlier. Those who took extra cash may now have to pay it back.
The machine gave £60 every time it was asked for £30, but also gave away more cash in different multiples depending on the money requested.
Some customers reportedly used several cards to withdraw money.
The Nationwide said it appeared the employee of a firm they use to top up the machine had loaded the notes in the wrong way.
A spokeswoman for the Nationwide said: "There was a problem. We are investigating what transactions took place.
"It is disappointing no one reported the matter earlier in the day when lots of people were aware and it is disappointing some people were deliberately using the machine to take extra money.
"Legally, recipients of payments made by mistakes are not entitled to benefit from it.
"We will reserve the right to contact people about the matter on an individual basis."