Okay, so for a change, I decided to embark on a river. This river had carp ad infinitum, a two z-level waterfall, and an aquifier with only one tiny square opening to the glorious rock underneath. (I use Reveal for all maps with aquifiers. Saves a lot of hassle.)
So after creating an awesome fortress with an entrance under a waterfall (Every time a carp came along, there would be a satisfying *Thud* as they landed on one of my walkways and died), I decided that a cool project would be to have a river-draining device that would dry up the river and refill it at the pull of a lever.
I dug down into the aquifier under the river, and constructed two bridges over it, connected to a lever.
Then, I sent a couple of miners to carve out ramps that would cause the river to flow into the aquifier.
Unfortunately, BOTH of my miners, after digging out the ramps, were caught in the flow, and fell into my aquifier. There were no ramps, as I had channeled (Note to self: Use grates). Both drowned almost instantly.
Of course, this was only a minor setback, I told myself. More miners would come. I could trade for picks when the caravan arrived. We would try again.
I went away for an hour, (All of my stuff was pretty much self-sufficient), and when I came back, there was one dwarf left, stark raving mad.
Of course, I checked my announcements to see where everyone had gone. Were there any carp attacks? Dehydration?
But no. All I could see was "Urist Mcdwarf cancels Store Item in the Stockpile: Dangerous terrain" times infinity.
Then it hit me.
I zoomed over to my failed river-draining project. And sure enough, THIRTY FOUR corpses had fallen into the same tile! Every single dwarf had marched to their doom, entranced by the promises of a copper pick, or perhaps a Pig tail Hat, and was swept away into the drowning chamber o' doom. The only dwarf left alive survived only by going insane before he too could commit stupi-cide.
It was as if every one of my dwarves had stood up one day, and walked, single-file, into a pit of 7/7 water.