edit: I'd include screenshots, but I don't have internet access on my DF computer. :\ (Pity, because with my processor, I can gen a world in 6 seconds.)
So I decide to build myself a new fortress. I search painstakingly through the embark screen for the most interesting terrain I can find. Lo and behold, after many long minutes, I discover an area with a chasm, a river, and a magma pool! Awesome! Only when I get there, I discover that there honestly isn't that much room for my typical style of fortress, and that chasm is much goddamned longer and wider than I thought it was. Oh well. I enjoy a challenge.
I spend a good hour setting up my preliminary food and defense systems, and I no longer have any room to expand horizontally. Since I prefer to see all/most of my fortress at once, I decided to go ACROSS the canyon and make myself an impressive series of bridges and fancy schmancy waterfalls. I completed all of the bridges to my satisfaction and was in the process of building an impressive residential complex, when my fishery worker decides that the best course of action in completing his duties is to believe that gravity has temporarily broken. Needless to say, he's probably still falling.
So a bit wiser, I start to build walls along the sides of the bridges to make them safe to walk. My masons, being of the less than intelligent sort, set about building the walls in the MIDDLE rather than on the edges first. There being absolutely nothing to hold the magical floating walls steady, every time they build a block it collapses and rocks the bridge, flinging dwarves to and fro. I was elsewhere at the time, and only found this out afterwards, you understand. The only thing I saw at the moment was a cascade of pink announcements and a cloud of dust when I got autozoomed to the scene. By the time I figured out what was going on, the only dwarf left was a farmer.
Moral of the story? I haven't yet learned my lesson and I'm reclaiming the fortress shortly.