Final Fantasy 6 (aka 3 on the SNES) - bosses are immune to instant death attacks like the Doom spell. There's a bug with the Vanish spell that clears this immunity. Vanish makes the target invisible, and normally there's no reason to cast it on an enemy... but if you do, you can then cast Doom and kill it instantly. Makes the most challenging boss battles in the game an absolute cakewalk.

I believe this was fixed in later versions of the game. I've only played the SNES version.