1. Tell them and give it back because you know they will be held responsible if their till is short at the end of their shift.

2. Say nothing and count your blessings, thinking it's their own fault stuff em.

Once many years ago in a Liverpool pub I was given change of £20 when I had only given the barmaid a tenner (I said nothing), the next week in the same pub I jokingly said to my mates let's see if she does it again and she did! To this day I still don't know why that happened unless she was on a scam amd mistook me for someone else?