There have been two times where I left a shitty tip. One was with a group of friends aged 15-17 and the waiter was being an asshole to us and nice to other tables. Only came to the table for us to order and for the check. I still feel bad about it because we left a stack of quarters that was probably a 5% tip.

The other time they undercooked my mom's food (who gets extremely sick if red meat isn't cooked thoroughly) and after asking it to be cooked more they just put a piece of cheese over the bitten part to hide it. It was cold when it came back and obviously not cooked. Waiter argued it was fully recooked when it obviously wasn't. We just paid for the food as it was $10 and I left a penny on the table because I'm a petty asshole.