I caught a necromancer and decided to use him to set up a training room. Dug out the room, dug out the pitting floor above it, made hatches, build fortifications, etc.
First mistake: pitted the necro too early, before I had hatches over the pits. Took me nearly a season to cancel and reassign the hatch jobs until my dwarves stopped cancelling due to line-of-sight into the necro's pit.
Second mistake: left LOS into my main courtyard. Dwarves would be running around, not even near the pit, and drop bins and stuff because the happened to glance down the ramp fifteen tiles away and see an enemy behind the fortifications. Took half a season to get barriers up to prevent that.
Third mistake: left general LOS into the fortifications. I was thinking, "I'll only ever have my military down here, no need for civilians, so it should be fine." Firstly, wasn't thinking about needing civvies to dump the bodies to prevent miasma. Secondly, every time a soldier decided they were going to drop some equipment in the pit and go get a new piece, a dozen civvies would run into the pit to stockpile the abandoned armor and then run away like pansies.
Fourth mistake: I didn't have any sure-fire way of getting rid of the necromancer when I decided that this had been a bad idea all along. I ended up killing him by pitting all of my large domestic animals in with him. A llama kicked his head in.
EDIT: Left the fort running while I went to do dishes. Came back in the middle of a tantrum spiral caused by troglodytes pouncing on haulers in the caverns. Various insanity took me from 60+ dwarves down to about 10, when a necro siege and two goblin ambushes showed up at the same time. The goblins that survived the zombies and my cage traps killed everyone but my most accomplished soldier, who was stranded in the lowest cavern with only one leg. He was starving, dehydrated, extremely tired and miserable, but still had the spunk to elect himself mayor and ban the export of rings. Shortly after this an anklosaurid FB showed up, ran to a spot three tiles from him, stood there for a day or so, then ran around the corner and killed him in one hit.