I was at a bar with a friend and the bartender made my friend the wrong drink. My friend said don't worry about it, now you're getting a negative tip, just kidding around. The bartender didn't think it was funny at all.
Can you do that on a tab when paying with a credit card? You know, under the space for tip write in -$20 and then the total with the negative tip? That would be funny. Let's all try it.