A long-ish time ago, probably not long after the euro was available as coins and notes, I read a story about a shop in the UK that accepted euros (more as a publicity thing than anything else).
A youth wanted to try that out so he changed some pounds into euros at the bank and went shopping there. They gave him change in pounds—the change was more than the sum of the prices (in pounds) of the goods he had bought! When he pointed that out to the cashier, she says it's what the computer told her to give him.
Turned out, later on, that someone had entered the GBP:EUR exchange rate the wrong way around, so the computer thought it was 1.4 GBP (or whatever) for 1 EUR instead of 1.4 EUR for 1 GBP (perhaps since most currencies are worth less, unit for unit, than the euro, but the British pound is worth more, so the exchange rate is usually x XYZ = 1 EUR with "x" being larger than 1 and XYZ being the local currency).
However, after he pointed out the error to management, they still didn't think it worthwhile to do anything about it. As I remember, the kid kept going back and buying more stuff and getting more money back. He figured after he had tried to tell them twice and they didn't do anything about it, hey, it's free cash.
Can someone find me a link to the story? I haven't been able to dredge it up.