Several related to customers at my job...
Item won't ring up? "...GUESS IT'S FREE!" *stupid laugh*
Drive-Thru is FREEZING cold? "Oh, I have exact change!" *digs for 30 seconds*
Bringing an item to the counter, deciding they aren't getting it, then not putting it back.
Leaving those little scraps of paper towels on the floor in the bathroom.
Leaving an insane mess of crumbs and napkins and things on the table after eating.
Paying for something that costs like $5 with a $100 bill.
Paying for something that's nearing $10 with change and less than half the change is quarters.
"Do I order here?" We have a window you see immediately after walking in with a big "FOOD ORDER" sign.
Saying they have food to pick up but not giving their name or even what the food is.