So I was mining out a nice vein of Adamantium, when shaboom! I accidentally unleash hell. Small facepalm there, should have been more careful. So I wall it off, sacrificing my two legendary miners to hold back the demonic horde. Then, having spent a load of time training up some new legends, and feeling smug and secure behind my flimsy chalk wall, I set them the task of ninja-mining the outside of the vein where the evil bastards can't get me. However the hauling was going a bit slowly due to my 1x1 100-odd-z-level shaft, so I decided to expand to a 3x3. It's when I get the notification that I've discovered a load of minerals, eerily reminiscent of when you break through to a cave...
That's when the true facepalm begins. First, my miners have indeed opened a hole into a chasm... one which is filled with demons. Secondly, they happened to dig into the bottom of the lake that lay at the bottom. So my two freshly trained legendary miners are promptly swept down the stairs by the rushing tide, down to my under-construction magma forge area, through the channel, and fall to their melty deaths. Along, I should add, with the 30 dwarves who happened to be down there tidying up.
To top it off, as the water goes down and drowned/smashed/pushed-to-a-firey-death all the poor buggers downstairs, the demons fly up, promptly tearing seven shades of shit out of my poor minions. One of them flew pretty much the distance from one end of my very long central corridor to the other. While wearing full plate armour.
Then, to top it all off, the game crashed. Apparently my computer felt enough was enough.