Fighting over how much to tip the waiter/waitress. Also fighting between servers over the tip itself.
"Lunch went well, except for the decision on how much to tip, which erupted into gratuitous violence."
Gratuitous violence is violence for the sake of being violent. Usually assocciated with low budget movies and cult videogames. Given without need, often unrealistic.
Grand Theft Auto had a Gratuitous violence bonus.