My strangest rules lawyering session was probably anthema to most.
I was at Dundracon with the rest of my gaming group and we had agreed to meet up in one of the rooms to patch together a d20 game on the fly. In this group there are at least 4 of us who have DM'ed, so I figured it would be fine.
Unfortunately, everybody wanted to play a game I had quit in disgust a few months previous. The DM of the game is a good gamer, but a whole bunch of little things finally collided to make it something I didn't want to continue.
Needless to say it was like past midnight, no registration going on and I didn't want to dig through the open gaming area for something, so I figured I'd try to have fun and play a new PC. Everyone else conviently had their old character sheets with them.
Instead I am forced to play an NPC. That was fine. Now, this game takes off RIGHT AT THE POINT I LEFT THE GAME, and suddenly all the bad juju starts flowing again. My best buddy excused himself and never returned, he decided he wanted to sleep instead and I envied him.
I made the decision then and there to play his NPC with the intent to get him killed. Now, I didn't want to be obvious about it. So, I rules lawyered in favor of the DM all night. I made sure to inform him when spells expired, when I was provoking attacks of opportunity, when others did the same.
I was being a little bastard. The game ends, nobody is dead and we retrieve the treasure and then at the end what does the DM say?
"I'm giving you extra xp for being so honest. Never have I seen somebody care so much about following the rules and I think it helped the game."
Somehow I was able to control my expression. Needless to say, they wanted to play again after that and I had better things to do.