I had an older lady and her grandson at one of my tables last night. She had a salad; he had some mac and cheese. They lady was perfectly polite, and normally the $3, 15% tip she left would've been just fine.
But the kid puked. Quite spectacularly, actually. The stuff was splashed all over the floor and wall of the entrance to the freaking kitchen. Thank god we have two doors so we could go around it, because it was just disgusting. In full view of most of the restaurant, too. And then he threw up all over the ladies' room, as well.
When the staff has to clean up your kid's biological waste, 15% is the equivalent of "fuck you".