There was a chasm a ways from my fortress, which was exposed up near the surface but a large cage that led to it. I'd dug a long corridor towards the chasm and was using it to dump captured goblins. Good fun.
Then, after a while I get a "miner has been struck down" notice, followed by a few more struck down ones in quick succession. A giant bat had turned up as my dwarves were dismantling the built and now empty goblin cages for reuse, killed 2 dwarves and a couple of pets as well. I make sure to forbid all of the corpses and items to prevent the wave of suddenly daft, stupid dwarves from dashing gleefully towards the killer bat for the chance at a new sock.
So, I turn my 3 units of 5 sparring dwarves, who're still recruits but armed with crossbows that they've trained with for ages, onto on-duty, set their patrol and stationed areas to near the pit and look forward to watching them shoot the bat to oblivion.
That, of course, doesn't happen.
It seems that, despite the training, and each being equipped with a good crossbow and a full quiver, which they have been practising with, they all suddenly forget how to shoot.
The first couple that try to fight the bat get instantly killed. Two more fall down the chasm. My one marksdwarf gets killed after loosing a single bolt by an enraged bat, and his war dogs soon follow. The remaining dwarves are slaughtered, one by one, running in a blind panic around the chasm while being chased by this singular bat, except one that just stands there staring into space until he, too, is slaughtered.
I swear, forbid EVERYTHING in the area (which doesn't prevent the deaths of 2 more civvy dwarves who decide to go after some dropped equipment despite it being forbidden) and lock my door to the area, then check my population.
Two recruits left. Two. Both of whom only survived by avoiding battle entirely by being asleep.