I've seen lots of interpretation of this. I've heard of all kinds of crazy things happening in sessions from a meta standpoint, i.e. crazy things players do (as opposed to characters.) One post on this topic I saw referred to a guy at a con game mooning the GM and doing a little dance of some kind for a roleplaying benefit, since his character was doing the same to somebody else. Another guy I know went to a game of folks he didn't know, and one of the guys there decided that he was LARPing in the middle of the session. And that he was playing a vampire. And that he was going to try and bite another player on the neck with his fake vampire teeth.
I consider it a privilege not to have any stories of that kind to tell. I do have some stories of crazy things happening, but it's always to characters not with players.
Which one to tell? I actually have quite a few, and narrowing it to the most crazy is probably not something I can do. Here's a pretty entertaining one, though.
Shautha was a character in a game I was GMing. She was a half-orc barbarian with a charisma score of... I don't remember, about 5 or so. Not an attractive character. She had the sex appeal of an angry shoggoth or Lena Dunham (but I repeat myself.) But for some reason, the player decided that she was going to play up this character's obsession with girly things. She really wanted to be a pretty little princess. Or something.
There was a great moment where they were in the house of a suspicious minor lord who they knew was up to no good, but they didn't know exactly what his deal was. So Shautha decided to flirt with him. I can't remember anymore exactly how it happened, but somehow, although initially horrified, this minor lord somehow decided that really beefy half-orc gals were his thing, and they retired (with another character, who was actually attractive) to his bedroom suite as a distraction while the other PCs searched the manor. Trying to lead him on without going too far (while Shautha on the other hand, horrifyingly wanted to see what a sexual encounter could be like) while the other characters raced to discover what ended up being the body of the patron that they were supposed to meet in town got really wacky really fast. When the characters finally busted into the room to confront him on the death of their patron, the one character--a normal human rogue--who had at first led the flirting distraction, was desperately trying to escape the room, while the NPC was motor-boating the half-orc and singing "Oh, you touch my tra-la-la" to her and bouncing on the bed.
Naturally, it didn't end well. The house ended up burning down, although the minor lord escaped thanks to his army of trained and armed gorillas (yes, this was the same game as mentioned in the post below) and the party was left with nothing better to do than attempt to rescue their characters, who as I recall, were carried off by demon lords.
Later, they encountered this guy again. He had been nursing a burning crush for Shautha even after everything, and tried to convince her to leave her group and run off with him. I think she almost did it too. Especially after they discovered a cache of bad poetry written about her.
So, anyway.... Most crazy I've ever seen? Probably not really. But it's certainly a crazy story, and it's the one that came to mind today.