Actually one woman tipped me some change on a $30 meal just to insult me, because it was more insulting than leaving nothing. She got really mad and said I was lying to her and I changed the price on the receipt by adding 2 cents to trick her (the computer rounds to the nearest nickel so we don't deal with pennies). She said she loved the food and my service but couldn't tip a liar. I told her the computer adds 2 cents and I didn't change it, but if she wanted I could give her 2 cents off, and it was okay if she didn't tip me, and she got even madder, and said "Why are you arguing with me!?!?" Then she changed her mind and gave me some coins, which she like smacked into my hand in a fit of rage. She actually told me to put my hand out, slammed some coins into it, and stormed out.
My boss was laughing at me so hard, and said I get all the crazies.
(And about a month later I saw the same customer in a movie theater telling the management she'd call homeland security on her neighbor.)