Oh man. I think my worst was as follows. I had built a little water feature in my dining hall, with sewers to drain it to the first cavern. A while later, one of my masons is doing... something... and decides that he just HAS to have one of the rocks left behind from digging out the sewer. So he goes down there, and of course loses his footing and gets washed out, dropping 6 levels to the floor of the first cavern, suffering broken bones everywhere and quickly drowning to death.
This is not the dumb part. After this happened I set up some vertical bars to prevent people being washed out, and forbade all the rocks in there anyway, but it's just the sort of thing you expect when playing with water features. The dumb part is this next bit. After a while the cavern started getting kind of flooded, which was annoying since the sewer outlet was basically right next to the cavern exit from my fortress. So I turned it off for a little while, and dug a shaft from where the water was flooding down to the 2nd cavern. Of course this would eventually cause major flooding in the 2nd cavern, but I didn't care because I wasn't using it for anything. I turn the water back on and it works fine. Now, some time later, I get the announcement "Urist McMason has suffocated." Just kind of out of the blue. "WTF?" says I. I go find his corpse: it's on the floor of the 2nd cavern, below where the new drainage shaft lets out. ARRGH. My guess is, he decided he just HAD to have the one rock left behind from digging the bottom end of that shaft, and goes to get it. And gets washed out, and falls 11 or 12 levels, dying almost instantly from having his upper body pretty much flattened to a pancake. Apparently, he didn't learn anything from his buddy doing the same damn thing the previous year. And neither did I.