Back when I was in the US I ran a weekly game at a local nerd store, since I was a regular and knew the owners they asked if I'd run introduction sessions for pf1e and rogue trader once a month. They did the same thing for Warhammer and MtG so there was something on every week.
Anyway everything was good, I was having fun running one shots as episodic adventures (think warehouse 13 with different characters each episode) until I had probably the worst party ever. All of these people bar one were over 26
Dude 1: A guy in his late 20s, he was obsessed with 40k, wanted to make a space marine, wanted to live out a power fantasy and would pout and complain when he didn't get his way or you even questioned him in any way. Also a bad case of main character syndrome
Dude 2: This guy had somewhat of a reputation, he played MTG in the store and more than once had to be told that he wouldn't be allowed in if he didn't shower. He would act childish and throw tantrums and his defense was always "I'm autistic it's not my fault" which as someone with autism I really hated, his mother would also come in over any issue and accuse people of being discriminatory against her precious son. They were both banned after a particularly bad meltdown over a yu gi oh game.
Girl 1: Massive chip on her shoulder, no idea why she came, she kept saying really racist things and pushing politics and stuff into the game that made me really uncomfortable. Just pointed out things she didn't like and didn't seem to have any interest in actually playing, in particular she accused the only other female player of being racist when she said she'd like to play an orc
Girl 2: Really nice girl, younger than all of us, her mom actually came up to me beforehand to talk about the game. Really sweet girl who just had some social issues, she tried to play mediator a lot and eventually left crying.
So what happened? Well normally I had about 10-20 premade characters that people could choose from and then they would play through a little tutorial as they were new agents. Before we even played there were accusations of racism towards me and some of the players based on characters, there was a mini meltdown that I wasn't allowing futuristic stuff (since this was pf1e) and me telling the owner he'd have to sit in just in case. When we did start playing it quickly devolved into dude 1 and girl 1 arguing about politics, eventually girl 2 gets screamed at and starts crying. Girl 1 is asked to leave by the owner and flips out causing guy 2 to melt the fuck down when she kicks his bag, Dude 1 decides that this is a personal attack and tackles guy 2 who is having a massive freak out.
Ended with the police being called, two people charged with assault and everyone except girl 2 being banned from the store.
To this day that has been the most messed up experience, and I've DM'd for highschoolers