Another large scale goblin siege, buttoned into my fortress very well, activated the alert to make sure no civilians would expose themselves to the bazillion of ranged goblins that appeared.
So far so good, got lot of caged invaders in my traps.
Then i noticed their crossbow wiedlers shooting massively and repeatedly to my entrance rooftop. Strange as i have not deployed my marksdwarves there, being outnumbered by those ranged goblins, it would have been stupid.
I gave a look at the rooftop and saw in horror that several dwarven civilians were there, dying and crippled by so much bolts.
And then i remember a year ago having extended the burrow to include the roof , and forgot after all the time that i always had this sector included in my defined burrow.
Quickly i edited the burrow and removed the whole roof access and saw what is surely a bug i didn't knew about :
Rushing out of my more limited burrow perimeted, i saw lot of my usefull and very competent civilians dwarves rushing to the roof to rescue the dying dwarves despite the zone is out of the burrow, getting shots themselves, crippled and killed by the rain of non stop goblin bolts, lost every single of my doctors in that, several legendary crafters to that bug i didn't knew.
So lesson to learn, always check if your burrow are correctly defined before a siege army is reaching the bottom of your walls, it takes some time for the dwarf AI to understand the new limits of a burrow.
Can't believe so many of my dwarves that were living since several years, that resisted titans and undead fell to a gameplay "oops".
At least it opened lots of new room for future migrant in my gigantic individual bedroom system (helps a very lot in avoiding tantrum spiral, as they get happy thought of seeing their own fine bed, coffer and cabinet, and it helps a lot too with those cloth they throw all over the fort, as they store it then in their own cabinet instead of letting them on the ground)