I'll start from the beginning.
There was a goblin army, in which I had my single macedwarf stand against on the bridge to the fortress. He killed them all.
(Also a Rutherer was caught in a cage, while a goblin was riding it.)
I ordered the bodies and clothing to be dumped in magma, however the dwarves didn't want to touch their clothes.
(Meanwhile, my macedwarf is in the battle room, battling prisoners.)
Seasons later, I noticed these clothes sitting there on the bridge, and I thought, "Oh, I wonder if this will fly away if I lift the bridge while it's on it..." (Meanwhile, my Macedwarf is combatting the Rutherer, and is taking a while...)
I went to pull the lever, however I had two, unlabeled levers next to eachother. One controls the main gate, and one opens floodgates in eight 3x3 pillars with magma in the center in the main meeting hall.
Of course, I pulled the wrong one, and waited to see what happened to the clothes. Nothing did.
Meanwhile, the meeting hall, most of my dwarves were standing around, wondering why it's so warm.
The many, many deaths created a tantrum spiral. About twenty dwarves went berserk, the rest either died or went some other kind of insane.
(Finally, my Macedwarf is done in the sealed battle room, and emerges to see the skeletons of hundreds of dwarves, ghosts, and a few wandering dwarves, either weeping or babbling.)
All but one dwarf survived.
Label your levers, kids!