Channeling. Of course.
This was an early fort of mine, I had dug most of it just one layer down, in soil. I was channeling a river to create a sort of moat for the fort, forgetting to check where my fort's tunnels were.
Cue river pouring into the dining room. But it gets better.
The river freezes. Because part of it was open to air (I had channeled it, of course), it created a half-semi-glacier thing in my dining hall. Which insta-froze and killed two dwarves.
Cue butcher going berserk and spending the next three months chasing some poor hapless shmuck around the countryside. Two other dwarves became melancholy.
This was literally in the first winter of the fort, so I only had fifteen dwarves or so. With two dead, one berserk, two melancholy, and the rest most likely to snap once the melancholy ones died or the butcher finally caught his victim, I abandoned.
Moral: check where you're channeling, always!
Related story: digging magma channel from a volcano. Miner who broke the seal catches fire and dies instantly. Didnt forbid his stuff, so another dwarf runs to save his socks. He catches fire but DOESN'T die instantly, much to my chagrin, as he sprints out of the magma channel, igniting grass and creating a roaring inferno that quickly burns up just about all of my embark stuff plus all the beds and barrels and bins and booze I had up to that point. Half the map met its end in an apocalyptic firestorm, and the other half, the three dwarves on the "safe" side of the channel, decided they'd rather throw themselves into the magma rather than live in this misery (they went melancholy).
What is it with me and channeling...