I fortunately didn't met people the like of Virel told us. I mean, especially the immature 35 years old guy who look after 13 years old girls.
The most "bizarre" for me have been:
-- A group of players who had mixed three different games together, with a complete nonsense game as a result. As far as I remember there was Runequest + AD&D spells + Rollmaster stuff creating PCs with totally clunky game mechanics. This was awful and I only played once with them. These players were cool, but I suspect they were addicted to marijuana, probably explaining how they came to such a kind of game.
-- A squatter. 15 years ago I was searching for a group of players, and thus put a message asking for new players on the relevant board in my FLGS. I met this guy, who seemed normal, plus two others. We had our first gaming session, but as it ended late, he told me that since he lived far away, it was too late for him to go back home. Well, being naive at this time, I did let him sleep in my appartment. The next day, speaking with him I learned that he seemed to have no fixed abode, but residing at friend's places and seemingly changing often. Fortunately he didn't call me back for a while. Then, one year later he calls me, and asks me about games, asks news, etc. I immediately lied to him, saying I had getting married and as a resut ceased to play AD&D. He didn't insist.