This was on an older version of the game, but the memory still sticks with me as my first instance of Fun. I started up a new fort, among my firsts since I learned the basics. I decided to experiment with a different layout than usual, where all the workshops were aligned in parallel rows along the length of a hall. No doors or anything to separate them, just one big hall full of shops. At the far end of the hall, opposite its entrance, was a recently-built well. I had already had some trouble building said well, due to some bad constructions orders and then the water freezing. But everything eventually panned out, and my fort was running smoothly.
That is, of course, until the werebadger attacked. I still have no idea how it got inside, but at any rate, by the time I noticed the beast it was in the midst of my workshop hall. Since I had yet to have any problems I was without a military force, and had to promptly draft as many dwarves as possible. They were mostly lacking in any martial skills and resorted to pummeling the thing, receiving surprisingly few injuries in return. The werebadger quickly turned back into its regular form, a human, and proceeded to sprint as fast as he could to the nearest exit: right down the well.
Though the water wasn't terribly deep, I suppose the injuries he sustained after transforming kept him from being able to swim. He died shortly thereafter, contaminating the well with his blood and causing a cloud of miasma to annually rise up and fill a large portion of the hall. I wasn't actually sure how to extract the corpse without drowning some of my dwarves too, so I just abandoned the place. I've been spacing things a bit farther apart since then.