Recently, I've been building a Human keep. All food stockpiles, bedrooms, training areas, etc are in the above ground castle, and only the quarry, magma forge, workshops are underground.
The castle is surrounded by a moat, and under the moat is the quarry. In the interests of safety during a seige, I was building walls to either side of the draw bridge, and one of the walls was not built before a supporting wall, and fell into the moat. This did not bother me at the time.
Maybe two days (In game) later, I realised that a lot of floors and walls that I was building in being cancelled due to 'forbidden terrain'. I checked around the fortress, and found no missing floors or anything. SO I checked the quarry, and discovered that roughly half the quarry was filled with water.
It turns out that the wall not only fell into the water, but actually punched through the bottom of the moat, and now the water from a major river was now flooding into the quarry. I evacuated everyone, and had to abandon a lot of space quickly, because if I didn't have time to plug the leak, and it was about to move up a level and flood the workshops.
I had to leave 5 humans below ground, otherwise we would have lost not only the 10 workshops, but also the archery targets, the ammunition, and the entire crafts stockpile.
The entire old quarry is now a breeding ground for fish, whch is the only good thing that has come out of the accident.
Who else has any stories of terrible accidents in their forts?