On holiday, I walked out of a shop with a handbag that I wasn't on the receipt, which I didn't check until we got to the taxi. I presented everything at the till, but they must have missed it, and just bagged it with everything else.
I wanted to go straight back in, but my partner convinced me that the shop assistant (it was a big company) would get in trouble. We couldn't really go back anyway at that point, but I still beat myself up about it to this day :(
Have you ever caught a snowflake on your tongue?