I don't have many horrible RPG experiences since I play with close friends. Once me and one of my friends decided to check out a game at a local gaming club. We were about 13 at the time (yeah it shouldn't matter, but it did, sadly).
The first session was just to make characters, which is usually a good idea. Especially in this case since we chose to never attend an actual game session with the gaming club. We didn't even make it thru the character generation session.
But that's getting ahead of things. We go there, get to making the characters. The game is Rolemaster, and the other players who were hanging around (two highschoolers + one player in his 30s + the DM a guy in his 20s) had already made their characters. The system was familiar to us, so we just started throwing dice and making the characters. The other players and the DM seemed to be pretty nice guys.
Then the character generation session started to go horribly wrong with my innocent question "So what characters did you guys roll up?" I just wanted to know so that we'll get a well rounded group. First the dude in his 30s pipes up:
Dude in his 30s: "I'm playing a half-ogre lesbian."
Us: ". . . " just unbelieving stare from us
Dih30s: "Yeah the DM allows half-ogres from this supplement .. I rolled that Ogre was the father, but we ruled that out as impossible.", taking the wrong cue and thinking we were unsure about the validity of his chosen race.
Us: ". . ."
DM, probably thinking that the look on our faces is again because of the H-O race: "Yeah, consider what would happen if an ogre had sex with a human female. The woman would burst! Heh, heh."
Us: ". . . lesbian?"
Highschooler: "Yup, and I'm playing a bi-sexual halfling!"
DM, getting in this know-it-all lecturing mode: "You see, nobody in his right mind would go on dangerous adventures. All the adventurers in my campaign have to be a bit mental; all the player characters are sexually deviant - it's very good for roleplaying. I've got these custom tables for your characters too, you can roll for your own sexual deviancy."
At the time I didn't even realize how wrong it was to label sexual preferences as mental illnesses, or how wrong it was to go into that kind of stuff with 13 year olds. I just changed knowing looks with my friend, and we started to come up with excuses for leaving "Um .. we'll finish these home .."
I've kept the memories suppressed, but that experiment killed RPGing with anybody but my friends for me. I've not played at cons, game shops, clubs, etc.. since the incident.