I'm currently watching my greatest fort ever kill itsself.
We'd survived countless sieges of hundreds of various modded and unmodded races without incident, had been progressing well, had finally set up a way to pass through my fortifications quickly without allowing invaders to bypass my traps, and just all in all kicked a ton of ass everywhere. Didn't even have any unfrozen water above ground, we'd been doing just fine without it.
A single deadly dust flying FB i'd forgotten about from early in my fort found a diagonal gap in the bottom of the execution drop i'd dug out to get rid of prisoners quickly and got in my fort. He set off his dust thing one time and didn't manage to kill anyone with it but himself. However, his dust had a rotting syndrome in it, and about 1/5th of the fort caught it before i realized what it was.I made a hospital at my front entrance so that my next siege would kill them and they'd stop getting miasma everywhere. This worked very well, my population dropped from 104 to 80 in about 20 seconds. Then i remembered that instead of my usual humongous meeting space that keeps people from getting to know others well, i'd gone for a 1 tile one to get a mood boost early. So, instead of nobody really caring who dies outside of having to be bothered to haul the corpse to a coffin, everyone is really great friends with pretty much everyone and the mood of the fort goes from awesome to batshit crazy in seconds. I realize that now all the animals have caught the rot and quickly throw them down the execution pit thingy, but it doesn't even matter anymore because people are going berserk, destroying stuff while tantruming, and running around naked babbling like a crazy person.
We're down to 40 now, despite a migrant wave full of fresh faced new workers that were immediately attacked by the mayor who was then seconds later killed by the only remaining sane military. They're all miserable now too for some odd reason.
I'm just going to sit here and watch it fall all the way apart.