I have a slew of horror stories, but they all revolve around one player. So I guess I really don't have more than one story as much as I have experience with one particular horrific player.
He was a munchkin to the Nth degree, the type that would min/max the hell out of his character, complain and throw an adult tantrum and sulk if he wasn't better than every other character at everything, and would generally bring game to a crashing halt if he didn't always get his way. I won't even mention what happened when he almost died once. He would whine and complain and argue with the GM non-stop until he could do whatever it was he wanted to do if he was told no. He always played the exact same character in every game. Same name, meta-type, load-out etc. Got to be really old after a while. Unfortunately he was just really too much to deal with outside of game and so we eventually just stopped playing with him. Now that we are restarting up the group (without him), the games are going much smoother and are way more fun. Go figure.