The forces of darkness arrived
So last night, my fort was in the middle of building a grand sewer system to provide water for all living areas, hospitals, and random pit stops in the caverns. All wells were at least 2 z levels deep, cisterns were fuckhueg reservoirs, all the water came from a river up at ground level to protect against random swimming evil baddies crawling into my shit.
Then the forces of darkness arrived.
A force of goblins and trolls, hell-bent on slaughtering my fortress to a man. They went after my under-construction "main" halls first, rampaging through the corridors, beating down my miners and random peasants. The miners gave as good as they got with their picks, felling trolls and goblin alike, but without actual armor they fell eventually, as did the rest of the dwarves panicking like mad within the cluttered darkness of the gigantic stone halls.
Then they turned upon the outpost I had set up at the bottom of the hill, barging in and beginning the slaughter. I had 3 squads of dwarves trained in swords, crossbows, and hammers, but they all fell to the onslaught of darkness. My numbers dwindled from 136 to 30, then slowly lower as the remaining dwarves panicked within the stockpiles. 2 had fled into the barely-completed but still cluttered sewers for respite, at which point I had a third dwarf pull the lever, sealing it off from the outside world. Unfortunately this also allowed the water to flow in.
"Urist Gonnadie has been unhappy lately. He has witnessed death. He was comforted by mist lately. He has complained of the nasty water lately." Eventually he and a lone peasant were the sole survivors, taking refuge in the sewers as the trolls and goblins ransacked my fortress. The water levels rose, and eventually the peasant drowned, to be followed by the mayor. Then, the game crashed the moment the mayor died.
I had not saved in 3 hours.