We were playing Rolemaster, and the mid-level party were on a ship that was also carrying a vampire as a passenger. Other passengers started dying, and the PCs had a mystery on their hands. This resulted in a fun couple of sessions of sleuthing and cat-and-mouse before the party eventually confronted the vampire...

...who stood up from his chair, gave a quick villain monologue, and (having won initiative) cast a spell.

I was rolling openly in front of the players, and the vampire rolled a fumble on his spell casting roll. Then he rolled open-ended on the fumble roll and managed to fry his brains with the backlash.

So from the point of view of the characters, they found the vampire who had been killing passengers, confronted him ready for a tough battle, and then watched as he simply keeled over and died in front of them without them having lifted a finger against him.