I still have this save around somewhere. Many years ago, I started a fortress in an interesting spot. There was a cliff with a river flowing over it, turning into a many-levels-high waterfall which fell into a stone area, surrounded by forest, at the cliff's foot. I built my fortress into the top of the cliff. The entrance featured a descending staircase to the west of the waterfall, leading to a stone bridge across its front (halfway up) that allowed dwarves to walk through the waterfall and be soothed by the mist, and the fortress proper on the right.
As winter fell, I noticed that the forest biome to the south was colder than the stone cliff; the water at the base of it froze at the border, but it remained liquid in the cliff biome. "Weird," I thought, and looked away to continue building things.
I looked back a few minutes later. The ice had blocked the river, which was overflowing onto the stone, and the wall was growing wider. Eventually the frozen flood encompassed the entire stone area at the base of the cliff, and the furrow the waterfall carved into. And the water started to back up.
And it backed up enough to overtop the ice dam. And froze at the top. And filled up further. The wall of ice grew taller and taller, creating an enormous vertical reservoir of water trapped behind ice. I had no idea how far this would go. I redirected every single dwarf at my disposal to seal the entrance bridge as the water lapped at their feet, walling themselves into the fortress. The water rose past it, and to the top of the cliff.
This status quo lasted the whole winter. For the hell of it, I kept the fortress walled up, its entrance underwater, and developed things internally, instead of building another way out. Nothing unwanted could get in that way.
And then spring arrived, and my computer ground to a halt as the entire wall of ice melted at once. The water, suddenly uncontained, fell into the forest below with a splash, flooding the entire thing and killing everything that lived there. This took several minutes of calculation as the game's water simulation ran at its limit.
This happened every winter. Eventually I built a drawbridge triggered by a pressure plate mechanism below the entrance; water rising high enough would seal the fortress off automatically. My best chef got smashed in the drawbridge mechanism but it was a small price to pay for safety.
