This one happened fairly recently. The party had gone through a large cave system clearing it of frost giants. After several sessions of exploring and fighting basic ones, we got to the BBEG. The battle raged on for several rounds, with the barbarian beating it repeatedly, and the rest of us either using ranged attacks or spells. I was playing the warmage in the party, and kept throwing spell after spell at the thing, hoping to get through his saves (the DM had given this one several class levels plus a template, IIRC). Finally, I simply told the DM to roll a nat 1 on his next save. Prismatic ray + 5 on a d6 (turned to stone) = Frost giant statue. The DM actually rolled a 1 on command... it was great. He tried telling us that the giant wasn't dead, just turned to stone. So our barbarian cut off the statue's head with her adamantine sword and the druid cast stone to flesh. Done deal.
That one was one of the best.