This thread's a bit tl;dr, so I've skipped a few dozen pages.
BUT
The greatest logic failure in any game, always is, has been, and always will be the AI in Dwarf Fortress.
Really, these guys blow my mind when it comes to decision-making.
Here's the scenario; I'd built up a nice little fortress, where my dorfs were living comfortably and happily. Of course, the main entrance was barricaded well enough, an the defenses were nice.
Inevitably, some of said dorfs came under attack in the 'outside world'. Thankfully, the swordsdorf escort managed to hold most of the nasty goblins back while I rushed the bunch of them back through the wooden door. Unfortunately, this raid had claimed the lives of 2 swordsdorfs and a civilian dorf, and their bodied lay at the front door, among what increasingly resembled a goblin holocaust. Goblin bodies everywhere, and 3 dorfs.
Now, content that my crossbos-dorfs had taken out most of the goblins and would easily mop up the rest, I want and made myself a coffee.
Unfortunately, when I came back most of my dorfs lay dead in my fortress!
So I went back through the logs, trying to figure out; why? Plague? No. Wait - murder? LOLWUT
It turned out that the loss of the dorf swordsmen (but apparently not the civilian) had really hit the dorfs hard. I mean, they weren't even really great, the swordsdorfs, just the usual medium-level guys.
Yet somehow the loss of two of them caused an unstoppable chain reaction of sorrow and anguish, which in Dwarf Fortress means murder and suicide. Their close friends had murdered and killed themselves, then their friends, than another guy who walked near them, and so on and so forth. This had compounded itself from 2 soldiers getting slain in battle to most of my population murdering itself in a matter of minutes.
Naturally, I did what any self-respecting person would do. 2cat, obviously, followed by a diversion of a raging river filled with dorf-eating carp, a dangerously large plume of purple smoke (hey guys is this stuff bad for dwarfs? lol) and a level on the other side of the map that caused the collapse of the rest of my place.
Not long after this, the one guy left who I used to pull the lever suffocated in the middle of the forest. No idea why, or what happened, but he did.