I was getting back into one of my older forts the other day, before I checked and updated to the new version. This was a 43.05 fort I played a couple months ago, I think. I'd had a bit of a break, so I was trying to ease myself back into the game and re-learning how things like mass-pitting works (not very well in this version, apparently).
Suddenly, combat messages were popping up. I checked them, and it seems that there was a pathway open to the caverns, right above my hospital / well setup. I'd built the hospital system partially inside the cavern and surrounded it with floors, walls, and ceilings. The opening was on the roof of the hospital area, where I'd kept my stone stockpile used for construction. Apparently, I'd figured that closing that hole was low priority because there were no ramps leading to that place.
There was a web-shooting forgotten beast with wings.
Oops.
It ended up slaughtering a number of the dwarves who happened to be walking down that central staircase. Luckily, when I noticed it, it seemed like it was being held off by one of the immigrant elf bowmen. He didn't seem to give a damn that he was webbed, since he could still shoot at the forgotten beast. Eventually, said elf was murderized by the beast, but that ended up buying enough time for me to send in my military. The wrong squad. The one with substandard and incomplete equipment because there were goblins, elves, and humans among them. Then I sent in the training squad of marksdwarves, and my main military force.
I was also dealing with a ~70 strong goblin invasion, who had a habit of entering my base in 2s and 4s through an unsecured part of my roof next to a hill that I hadn't been able to recognize.
Eventually, the goblins got bored, about a dozen of them were killed or captured, then they left.
At the end of the session, my fort's dwarf population went from around 136 to closer to 86. One of the casualties, to my dismay, was my Smith, who was responsible for making my weapons and armour. Now I'm stuck with a relatively untrained smith that I'm going to try to train with copper / bronze before I let him touch my steel.
I've also reorganized my military by race. I have a full melee squad with my best gear and a secondary training squad with lesser gear as I gradually arm up. Both made entirely of dwarves. Ditto for a pair of marksdwarf squads. The immigrants with Bow skill get their own squad (might as well use their weapons and all those arrows I seem to keep picking up), and every non-dwarf melee unit is put into a suicide squad with whatever armour and arms they can scrounge up for themselves and will be sent to the front line as much as possible.
I want an easy way to find the "unusable" armour and clothing to designate it for melting. Searching for armour by size doesn't tell me what I can equip my military with, so I'm thinking that mixed military units might be more trouble than they're worth unless I use them as disposable conscripts.
Anyway, the short version is that poking into an old game and stumbling into two problems at once shortly after I got into the game led to a massive population drop and the death of that fort's smith.