It seems appropriate that my first ever Bay12 forum post should be a face palm moment, knowing the reputation of this game.
My last fort, Cryptyouth, was attacked by a marsh titan which mauled many livestock animals grazing outside before heading for the fortress entrance. Some military dwarves were gathered inside, and the hallway was lined with cage traps. I didn't expect to cage the titan - the cages were to catch keas and the goblins which never came - but I didn't expect what happened, either.
The marsh titan was a webslinger, and coated many of my dwarves in web, stunning them. As it charged through the fortress entrance, it triggered the cage traps, which had no effect. 4 dead dwarves later, a swordmaster struck it in the lower body and the severed part sailed off in an arc, slaying it. (That steel short sword must not have been so short after all). As the mess cleared and some tantrumming marksdwarves were dragged off to jail, I noticed a large number of "dwarf (♂) cages" in my animal stockpile, and more lying in the entrance hall. Sure enough, much of my military and an unfortunate baby had been caged by the falling traps. I was unable to pasture the dwarves, so I tried building the cages and unassigning them, but not all would construct properly, or so it seemed at least. I should have tried to sell some to traders for !!science!!, but eventually I got them all out after several months of confinement and occasional water brought by dwarves with nothing better to do, like storing an item in a stockpile, attending a party or going On Break from a party. The dwarves seemed to see no difference between their own caged friends and the gorlaks we had trapped in the caverns below.
This is a tie with the time an unfortunate web collector was mauled by a molemarian in a cavern and was left to suffer infection and injury while 1 doctor partied, another went On Break and a third insisted on storing items in stockpiles until I conscripted a farmer to do the job. The web collector, unsurprisingly, died.