I just had the most comical accident that I've ever seen in Dwarf fortress.
In order to properly explain what went wrong, I'll have to first explain how my defense system works.
My Fortress is underground, entered via a five-tile wide tunnel driven horizontally into a steep cliff face. I've built a castle just outside my entrance, with a gate-house surmounted by fortified battlements.
Hostile invaders will first have to survive enfilading fire from my Marksdwarves positioned behind the fortifications on top of my wall, then force their way in over the drawbridge (which I usually leave open), through a field of cage-traps, then cross the outer compound (which is where my Melee Dwarves train and practice), and then force open the doors to my entrance hall.
The entrance hall itself is deadly enough... it has several more rows of cage-traps, dry-moats on both flanks and at the end of the hall, behind which are fortifications screening three rooms for Archers. This allows me to bring enfilading fire on anyone trying to cross the traps.
When enemies attack, my first response is always to order the civilians indoors... to prevent them from suicidally running out into the fray to grab the socks or weapons dropped by a slain enemy. Unfortunately, when ordered indoors, the civilians tend to pack themselves into the trapped entrance hall, congesting that area and offering easy targets to any enemy who penetrates that deep into my defenses.
I decided to
improve my defenses, by channeling away the roof of my entrance corridor... thus converting it to "Outdoors", so the civilians would automatically flee
deeper into the fortress (at least as far as the stairway) when the alarm sounds.
Unfortunately... I made a tiny miscalculation when designating which areas to channel out.
Picture this: My Legendary Minor is eight levels up the side of the mountain, chipping away the roof directly above the trapped portion of my entrance hall. Unfortunately (and unknown to me), he is standing on ONE roof tile, and happily chopping out the only tile that SUPPORTS it.
A caravan has arrived, and my civilians are hauling goods out to the depot. SUDDENLY... the enemy attacks! I sound the alarm... Military Dwarves run out to man the castle walls, all civilians run indoors! Just as my civilians are flooding through my entrance, my Legendary Miner succeeds in chopping away the last roof-tile that supports the section that he is standing on... the entrance hall roof collapses!
Carnage ensues...
The Legendary Miner, surrounded by a seething cloud of collapse debris, plummets eight levels down and lands directly on top of the struggling mass of civilians headed inside and Military Dwarves headed outside... JUST as they are all crossing the field of traps!
Luckily, his fall was broken by something soft. Unluckily, it was another of my dwarves. Both are knocked unconcious, as is one of the visiting caravan guards. All three fall into my own traps and are caged. My Legendary Weaver is blown sideways off the entrance walkway and falls into the haha (the dry-moat). He is trapped at the bottom... naturally, there's no way out of my moat, since I expect only enemies to end up in it.
While all this is going on, my Marksdwarves on top of the fortified outer wall are picking off the attackers one by one... none of the enemy manage to penetrate my gate-house and get within firing distance the scene of mass confusion in my entrance hall.
After breaking the siege, I start cleaning up the mess... I dig a tunnel into the haha to release the trapped Weaver, I set up the three cages containing trapped dwarves and connect up a lever to release them. Assessing the damage, the Legendary Miner has suffered a spinal injury, the Legendary Weaver has a broken arm.
No fatal casualties. All enemies killed. It could have been worse...