I've had two people in the last two days who just don't quite get how the whole "paying the bill and getting change" thing works.
The first was when I delivered a pizza to a woman. Her total was $15.85. She handed me two dollar bills and said "keep that." Then she gave me a twenty and said "give me change but keep a dollar." Okay ....
The second was possibly even weirder. This rather old gentleman paid his bill with a twenty and a five. His total was $21.11 ... and he told me to give him back one quarter. His son was as baffled as I was and said "Just let her keep it, Dad." But the old guy shook his head and said "No, it's okay." and held out his hand for his quarter.