One particularly terrible thing about tipping culture is that it allows customers to be petty. If some entitled person eats at a restaurant and not everything is to their liking, they can hit their server where it hurts when it comes to the check. It might be bad manners to skimp on a tip, but sadly, that's exactly why some diners like to do it.
This is something that has prompted intense discussion recently over on the r/Serverlife subreddit. One server shared how they had worked a particularly busy shift, leading them to forget about one of the tables in their area. In retaliation, the customers attempted to give them a negative tip, by writing out their own discount on the check. The post inspired some controversy, but most were agreed that it was an unnecessarily mean move, even if they had received bad service. It also seems excessive, considering the total was a little more than $30.