Our group once encountered a Rakshasa and his minions. He was way beyond our pay grade and the GM expected us to turn around and get the heck out. He was our BBEG at the time. Our sorceror remembered that we had a scroll of sleet storm and cast it between the Rakshasa and us. Then our Dwarven Monk ran around it and grappled the Rakshasa, preventing it from casting while the rest of the party dealt with the minions. When the combat finished the GM told us the session was done because he hadn't expected us to fight, let alone win and he had nothing prepared for that. We all had a great time though, including him.
One character I had potentially had the opportunity to break the game. We had just become demigods (back in 3.5) and I realized that I would potentially gain the ability to turn any metal into another (potentially unlimited wealth). The GM okayed it with the warning that spending too much might draw unwanted attention. Sadly, we never did continue the campaign so I've no idea how it would have gone.
On of my favourite derailing a BBEG encounter was in my first campaign. I was playing a Psion Uncarnate (3.5 again) and our group came up against some kind of Were-Boar giant thing. This GM I later learned is a) a pathological liar and b) always wants to win, so he will cheat. The big bad shows up, we're trying to figure out what the heck to do and one of the other players leans over and asks, 'you have ego whip right?' I went incoporeal and was able to do 1d4 charisma damage a round against this thing. In rage it tried to lash out against me but couldn't hit me and a few rounds later it was comatose. That was the day I discovered how crazy ability damage is.
