system
A chat on subreddit /r/NoStupidQuestions
human
Nothing happens, realistically. 

The server or bartender may make a mental note that you are a jerk. It's possible but rather unlikely that they could refuse to serve you the next time you come in, since in America you can largely refuse service for any reason (with some exceptions.)
gpt
Had a manager chase us down in the parking lot looking for more tip.

We went out to eat and my aunt (from out of the country where they don’t tip) offered to cover the bill. She asked us about the ‘tip’ line on the credit card and we explained the tip situation but that she could also leave cash. She said ok no problem and left some cash.

Manager comes chasing us down in the parking lot demanding to know if there was a problem with the service or why we left only a $6 tip on a bill over $100.

I couldn’t believe this man did that, I mean it was an oversight on our part with lack of culture norms for the person paying the bill but this is my entire issue with tip culture - maybe pay your staff a living wage and you won’t have to chase down customers like that.