well, to recount some of my more interesting fubars....
Magma Imp stories.
A) embarked, right ontop of the magma pipe, all the way across the map from where I wanted to start the fortress, only two miners survived the forest fire, nothing else.
B) Fortress2, embarked, saw an open mamga vent. Much more favorable starting position, was able to dig in before the imps started their shenanigans. Lost two dwarves to an unnamed attacker (bled to death), likely imps. Trained up two marks dwarves to deal with the blighters, but due to being two dwarves down, I was short staffed. My first dwarven Caravan appears, and walks smack dab into a forest fire (note they spawned on the side of the map, and happily trudged through the flames to my depot. None of the local dwarves died (they were inside). The fire started because an imp decided to fireball a groundhog. So I lost my first caravan and my trade liason, but on the upside I got my anvil for free, and the forest fire cooked two kolbold thieves.
Engineering Blunders
A) In one fortress, before I developed a taste for over-engineered magma/water works, I tunned to my magma pipe (it was capped) and built an obsidan factory on site. The design was relatively simple. I diverted water, via channel, to the factory from a nearby brook. This also served to power the magma and water pumps. The back wall of the factory was right on the edge of the channel of water.
Well... this was my first obsidan factory, and I built it two stories tall. The factory worked perfectly, at least on the first run (i was tunneling out the obsidan after it formed) I found that tunneling didn't remove the floor above, ergo it ruined the water mechanism. I cursed my short sightedness, and started to renovate my factory to function again. During the renovation process there was a collapse of a floor tile that dragged my ledgendary mason, a Master Miner, and my Baron into the water below. Well.. the mason and the miner both made it out of the water via a nearby ramp from the channeling process (I added the ramp because you know how miners love to jump into channels.) so only the baron died.
B) My last fortress ToolCrews, was going to be a constructed volcano. I was busy working on the internals, when I decided that the kings room should have a magma fall in it. So I wanted to test out the magma fall prior to throwing any dwarves in there, and I accidentally caused and overflow of magma that fell down my main access shaft (it wasn't a torrental flood, but a steady drip which was more than enough). The access shaft ran all the way to the bottom of the fortress, and was the only way to get up to the dining room, and most of the lower fortress (workshops and storage) suffice to say, a lot of dwarves died to super hot lava sauce before I could hammer out a secondary access shaft.