We (a party of level 12s) were fighting and undead roper (we're playing pathfinder but the DM likes to include modified beefed up 3.5 stuff) and my arcanist facing the prospect of being grappled, drained of strength and eaten (the gunslinger had already taken 60 points of damage in one attack from this thing) panics and throws a fireball. At that exact moment I, the player get struck by a feeblemind ray and decide that it would be a good idea to confuse radius with diameter and catch two other party members in the blast.
My this wasn't just any fireball, this fireball was cast using a powerful custom metamagic rod given to my arcanist in the previous session by a god. This was an ascended (making the fireball a mythic spell despite not being a mythic campaign), maximised, empowered fireball. The roper was vulnerable to fire so after taking 300 points of damage was no more, the swashbuckler saved and just survived taking 75 points of damage , the heavily wounded gunslinger did not.
My horror (and embarrassment) changed to confusion then gave way to relief as the body of the gunslinger shimmered and transformed into that of an undead ogre mage (he'd been transformed into a half-undead a while back so him showing up as undead when we cast detect undead was expected). The gunslinger had been killed a few sessions back but neither characters nor players knew he'd died and was replaced with an ogre mage. The player who's gunlinger was killed was waiting for the signal from the DM to destroy the party by casting prismatic sphere so we were really quite fortunate that I made a stupid mistake and wiped him out.