You know, it's possible also that this was fraud. Someone I know goes over her credit card statements in detail monthly, and she noticed a gas station amount that was off by $20. She went to the bank manager, who looked at the surveillance tapes in the store. Turned out that the cashier was adding money onto customer receips and pocketing the difference. (Sorry, I don't know exactly how it was done, but I do know that this actually happened.) I'm sure there are real errors also, but it's important not to rule out theft just because it's a credit card.