I did have that once. My digestive tract was warning me. I assumed it was too many chips.
In my first campaign GMing I had three tried and true players, all friends. Through the various sessions I'm being told of this mysterious guy named Scott. He is according to the two players who know him a complete freak.
But he's into D+D. They play games with him occasionally. One of them does.
Feeling left out, I induced them to actually invite him along to my game. I should have taken that look of caution between them and the subsequent (and frequent) warnings to heart.
Well, anyway, he gets invited along. Initial insults aside from my two players (I really didn't expect that from them.) I let him create a character and start playing.
Evil drow fighter/rogue. Righto, says I. After he attacks the party - about ten minutes into the game mind you - and they wall of ice him in a room to discuss the situation. He cracks a




and storms out. Comes back ten minutes later, apologises and wants to make a new character. Who then tries to assassinate the sorcerer who wall-of-ice'd his old one. The actual plot of the game was suddenly lost in a rather heated in-character mission to slaughter the new PC.
Anyway, assuming that he isn't here to ruin my game I take him aside and explain things, such as the point of role-playing and what not. I soothe the rather heated discussion and keep letting him play.
But it gets worse. The more he tried to munchkin his characters up by breaking the rules the more he got upset when he didn't get the character. (Me: Create a first level character. Sc: Here's my 27 intelligence wizard. Me: wh!?) He never roleplayed and while he never attacked the party again he'd often just wander off by himself. Well, I stopped inviting him to games but he somehow always knew when they were on. When he was there (It was an hour-and a half trip for him to get to the game place) I couldn't send him home, being weak willed. It resolved itself when one of the old crew started GMing and he left for another city.
The sad thing was he kept on unburdening his problems on me. I couldn't even stand the rancid milk breath on him and it was all I could do not to be harsh.
It was more sad than anything. But I just couldn't stop him arriving at the game and when he was there I couldn't send him home basically.