In a Powers & Perils (
www.powersandperils.org for the rules, if you're interested ) campaign I ran a 'while ago' (maybe...12 - 15 years?), the PC's were tracking down the secret lair of a clan of goblins that had been surprisingly crafty and successful at raiding. After multiple play sessions of searching, questioning, and general wandering about, the party finally found the secret hidden lair. The Goblin King was a warrior-shaman kind, of significant intelligence and charisma. Anyway, at the end of the session, the PC's had been successfully suckerd into a bottleneck trap by the goblins! The goblin's in hiding waited for their Goblin King to unleash his magical might! Initiative was determined, the Goblin King started chanting his spell...the PC's presseed forward (as expected) only to be shot at by bows, spears and daggers. The next round the Goblin Kings spell (a lighting-swarm) is finished. I roll for casting success...100. Abysmal Failure.

Rolling on the Abysmal Failure chart...13..."Loose 1d100 Energy Points". Ouch. Rolling d100...98. This reduces the Goblin Kings Energy level to somewhere south of -50 or so (at 0 you die). O_O In a blinding flash of internallized electrical energy, the Goblin King explodes on the spot. ... The goblins then stare at the PC's for a round...and the PC's stare back. Dead silence. Then the goblin warrior champion steps up and, in halting speech, says "Uh, we stop now. We go away. Not bother humans again. Deal?" The PC's agreed, and the menace of the Goblin King was at an end.
Funny and unexpected. And only because I let the dice fall where they may.