In one of my latest fortresses, I was making an above-ground barracks/fortification tower/megaproject. I also built a moat.
So in the middle of the construction, I'm suddenly not happy with my bridge roofs (roofs made out of bridges) because I realize that they won't show up in 3Dwarf. So I deconstruct them and a bit of the flooring in between them.
The floors crash down, through the 3 z-levels of the above-ground fort, through the moat through the food stockpile, through the bar/block stockpile, and into the great hall.
A dwarf dies in the process, and the recently-opened lava channel is thundering towards the new hole in the moat.
I don't know how I did it, but I managed to seal off the moat and rebuild everything before the lava death arrives, at the cost of the food stockpile.
4 in-game months later, I accidentally cave in a cliffside.
It, again, punches through the lava moat, into the refuse stockpile, and ends up in the weapon stockpile. Only this time, the lava moat has lava in it.
The firey death invaded my fortress. I just gave up, dug direct passages to the magma pipe, sealed off the exits, and watched as my fortress descended into madness. A dwarf went insane from the get-go, tore apart a child, and threw himself into the lava.
The rest of them huddled in the main hall, maybe thinking that the lava would not reach them.
It did. It killed them all.
Except for one dwarf, who went into a fey mood and managed to get into his room a second before the lava death sealed it off.
He ended up starving to death shortly after.