The cashier handed you an extra ten. That’s not luck. That’s a test that came with a receipt.

Finding yourself better off through somebody else’s error puts a decision in front of you whether you want one or not. Keeping it requires no action at all, which is the trap, because doing nothing feels different from taking something even when the outcome is identical. The person behind the till may well be short at the end of their shift.

Count your change before you walk off, and say something straight away when it is wrong in your favour. One sentence does it, and they will be relieved rather than annoyed. The same goes for a wrong bill or a double refund.