Poll Retail "Humor" Mine is a tie between "I just printed it this morning!" when I counterfeit-check $20s, $50s and $100s, and "what about tomorrow?" in response to "your total comes to $____ today."
EDIT: God, how could I have forgotten the one
birchsalt mentioned? When an item doesn't scan - "that means it's free, right?" Yeah, go ahead and try to run out that door with a $1500 MacBook right in front of security. Go ahead and do it. Let's see how funny that is.