This is quite recent: We had to recover an artifact in parts (a mighty wizard's staff), and one was in Skullport (for those who don't know the Realms: It's an underground town below one of the largest cities on the continent, and all kinds of evil creatures live there - mind flayers, beholders, duergar, drow, green races...) We have to run errands for mind flayers so they give us the information we need to find the next piece, and my LG cleric of Moradin is furious about it (only the fact that the people they sent us to kill were bloothristy madmen themselves made it bearable for him).
So after we defeated a mad halfling rogue we were sent to kill (which killed our big tank, so only my priest and the dwarven wizard were still standing), a high-level drow cleric (we knew he was considerably more powerful than us. I think he was level 19, and we were level 12) shows up to take the dead halfling away, to "revive him, he's such a funny fellow." When we insisted he let the murderer there since we needed him dead, he started to suggest that he would consider it after we ran some errands for him.
That was too much for my almost paladin-like priest. He told the wizard that he didn't expect him to join this fight, but that he had to do it - better to join Moradin today than Asmodeus tomorrow - and began attacking the priest (with the wizard's help, who didn't flee).
It was a going badly for us: We hardly hit, or managed to get a spel home properly, despite his magically lowered SR. So the wizard took desperate measures: One of the pieces of the staff we already recovered could cast imprisonment 1/week at CL 19. It still was a 1/20 chance (as we were sure the drow had to roll a natural 1 to fail the saving throw), but at least it would deal with him if it got through, and it had better chances of getting past the SR.
We used it, got past the SR, the DR made a roll, looked at us - and lifted the DM Screen, showing a d20 with a "1" on it!
That got us up two levels all at once (we ignored the "only 1 level" rule since it would be dumb waiting for that one XP, but we didn't get more than just enough XP to level up twice)
A great victory for justice!