So I was working away on my fortress, having had a surprising amount of success so far. I start digging a well shaft, with one dwarf tunneling straight down the series of channels. Meanwhile, I'm trying to figure out why my other two miners aren't hard at work constructing my underground resevoir I had all designated.
"Urist McBrilliantMiner has died after colliding with an object."
Huh?
I zoom down to the site, and see the original miner doing just fine. As I'm about to write it off as a glitch, I find my other two miners. Apparently they decided to leap down the single tile well shaft, about 7-8z levels deep, some time after my first miner had successfully tunneled to the bottom. Near as I could tell the first one to land survived without serious injury until the second one landed on him and killed him. Said second dwarf was badly injured but didn't bother to stop working. *facepalm*
Then there was the time where I thought I'd done so well in finding a spot where I had successfully tunneled down beneath an aquifer, where I started building my fort. I was close to the left edge of the map, so I started stacking stone storages to get stuff out of the way and make sure I don't miss out on any ore. 'Stacking', of course, intended to imply that I was putting each new one on a layer above the last. I facepalmed hard when I realized I'd managed to tunnel a straight up/down stairway right back into the aquifer I thought myself so clever in avoiding.
And the time where my expedition leader mysteriously died of thirst in a tunnel way up north on the map. I couldn't figure out where she was for a long time, mostly due to not thinking to check the tunnel that was perfectly accessible from multiple points. There had been no cave ins, no sudden bursts of high pressure water (in fact, no water at all, at that point - just plenty of booze). My best guess is that she was the only dwarf to have encountered the mysterious invisible troll that had destroyed one floodgate and my farm plot without ever being seen or interfering with any other dwarf in a way that triggered a message. The facepalm mostly comes from the fact that this dwarf was also my broker, and that the outpost liaison had been unsuccessfully trying (by hanging out in the main section of my fort, apparently) to hold a meeting for a few months until she died and the liaison left angrily.
Edit: This morning I had an embark where, as soon as the spring thaw hit a few minutes after I started, I discovered that my wagon was half-over an obscured murky pool that had been frozen solid when I started. Queue having to drain the pool in an effort to recover over half my supplies that sank straight to the bottom when it became liquid.