But during all this my traps (serrated disks) caused a piece of a random zombie to land on a wall, where it was revived. It can't get down, but I can't get to it.
Ahaha this just happened to me as well. At least, inaccessible undead parts.
I was busy trying to kill a vampire by cave in as a punishment for killing my legendary grower. This was on a neutral aquifer map, which required me to use the ground level as the cave in roof. The vampire escaped because I messed up with the generated ramps. I thought that would only be a temporary set back (just costing me another dead), however, moments later (presumably by coincidence) one visible necromancer showed up. At this point I was a lot more vulnerable already than I would've been under normal circumstances with all dwarfs inside, so I decided to go for an active defense.
There was no military yet (yeah, I'm really rusty since I only got back to DF recently, nor did I ever play much compared to reading forums to begin with). I drafted the whole populace of 60 dwarfs - children and sent them to target the necromancer specifically, instead of the Elf merchant skeletons re-animated by him in my trade depot. They nailed the necromancer quickly, and after that managed to kill the skeletons as well. Cancel order issued; mission accomplished... I thought.
Then I noticed most dwarfs were behaving weirdly - they appeared to run back and forth, away from my pasture enclosure wall. I checked unit list, and a partial skeleton was listed as not deceased. I couldn't see it however, and couldn't zoom it until later on. When I finally could, I noted there was an immobilized skeleton finger 1 Z-level up, unable to jump down from the wall. Nor could my dwarfs reach it when I re-drafted them.
I tried to get my dwarfs back by digging, however, with a river in the way it was easier said than done. My only miners on the correct side decided it was a good time to get food and drink. As a desperate measure I drafted everyone else as miners, but they all refused to work, presumably due to tiredness and job cancellations. I tried digging from the other side; the dwarfs back in my base would also disobey however. Potentially the problem was that they wanted to go up stairs to be scared off by the skeleton finger again, so I tried deconstructing the stairs, to no avail.
Meanwhile, the vampire was feasting on dwarf children down in the fort judging by the announcements.
From there, things were already pretty bad (I'm sure a more experienced player would've found a way to solve it at that point), but it got worse. All hell exploded when another necromancer showed up (I think). For a prelude, my 30+ animals in the pasture got massacred, by some other skeleton parts thrown over the wall earlier, now re-animated. Then the worst slaughter I have seen so far in my own games commenced in the halls down below. Let's just say my trap corridor wasn't particularly useful, and I hadn't prioritized building a physical obstacle. I think I went from 60 to near zero dwarfs in 60 seconds real time.
However, at the end of the carnage a single dwarf was standing: The vampire (mayor (elite hammerer)). The avenging undead and elves had gotten the better of me. Such beauty and elegance in intricate game mechanics.
This situation would've been completely avoidable if I had prepared differently/more, or for that matter reacted quicker to the cancellation-fest caused by the skeleton finger on top of the 1 Z-level wall.
Losing is fun.
Edit: Spelling fixes, playing until 6 AM in the morning has that effect.