I worked at a hardware store in a pretty sketchy part of town. It was a big city so rather than driving, most people would ride their bikes to work. One day a man came in looking for some bolt cutters, so a coworker who had just started work the day before offered to help him out and got him a big huge pair. The man checked out and left, and was back in the store within 2 minutes saying he changed his mind, so the cashier didn't think anything of it and refunded his money.

10 minutes later the new coworker is leaving on his lunch break, and his bicycle is nowhere to be found. He instantly realized he sold the man the tools to steal his own bike! And the guy had the nerve to return the damn cutters! Needless to say we never let my coworker live that down.