This just happened to me.
So I made a very twisty hallway between my fort proper and the caves, filled it with roughly 45 stone fall traps, put bridges at both ends of the hallway, and to set my plan in motion I mined away the last rock between the hallway and the cavern. Several "What THE SHI--" moments subsequently occurred.
1. The miner moves slowly along the hallway. The Forgotten Beast-- Sutu the Deep Barbs, a towering sauropod--moves in to attack the doors I set up as building destroyer bait at the end of the hallway. Turns out, he moves much faster than a dwarf. By the time the miner is almost home, Sutu is nearly upn him. And hey, what the--
2. The stone-fall traps aren't working. Not a single one activates. (In retrospect, I should have known, really.) Luckily the miner escaped, but what the--
3. A farmerdwarf enters the hallway. Oh man, I thought I turned off "auto collect webs," but apparently not. Get the heck out of there so I can raise the bridge, fool! More dwarves coming? What the--
4. As my oblivious dwarves retreat from the freaking brontosaurus, they open the doors, allowing the Forgotten Beast to move through some of them. But then my dwarves escape, so the potential disaster is averted for now. Please, Armok, let them pull the lever in time!
5. Some blue dwarf starts going into the hallway again, and is attacked by the beast. Oh well, it can probably get away, and even if it doesn't, surely it's not important. Hey, when is that bridge going to raise? Get out of there, Dorf! I don't think it's going to mak--
"Rakust Sakzullam, Stonecrafter, has been crushed by a drawbridge."
Rakust Sakzullam? You mean the legendary stonecrafter who has single-handedly produced perhaps nine-tenths of the exported wealth of this coal-less, iron-less, entire fort? THAT Rakust Sakzullam?
...
WHAT THE SHI--The thing that puzzles me the most is why she was there to begin with. She was set up in a craftsdwarf's shop on a layer full of marble rocks. The path between her and the nearest marble was much shorter than whatever was in that hallway. She can't have been trying to collect webs. Was she trying to get some gabbro? Just going for a stroll in the cave with the Diplodocus?
Sigh.