I've lost a few forts to stupid mistakes...
Once I tried to make a magma channel. I bumped into warm stone, made a 2-wide channel, then asked a miner to channel the wall to the pipe. I knew magma didn't follow pressure, so I was rather surprised to see my miner burn up. Some time later I realised that I asked the miner to mine the wall above the channel, and that magma will in fact not follow pressure, but will still fill, albeir slowly, that entire fortress section. I had no means to stop the magma at the time, so I abandoned. It took some 8 more tries to dig out that channel and keep the miner alive at the same time.
There were two times when I did some stupid things with flooding.
Once I tried to make a waterfall in my dining room. It was an ambitious project, the dining room was nicely outfitted with statues, and the water was supplied from two levels below the dining room, while power was transferred from some 3 levels above it. The whole construction was rigged to two levers behind the waterfall, one linked to the waterfall drain floodgate, another to the pumps' power supply.
So, I rigged it all together, sealed the machinery up (it needed to be watertight), and ordered the pump lever pulled. Yeah, it all worked fine for two seconds, before I realised that water hitting a down ramp (that formed a sort of emergency escape route out of the fort) also split in all directions, and that my dining room was flooding. I hastily ordered the lever pulled again. Some time later, someone actually got round to go and pull the lever. However, at that point I realised I ordered the wrong lever pulled! The drain floodgates closed, and water quickly filled the room. All machinery was flooded. Levers were flooded. The only way to enter was dismantling the first in the series of pumps that supplied water to the system, and allowing water to drain - but the system was designed to operate in winter, and with floodgates closed all water remained inside. I considered risking a miner and penetrating the outflow channel, but decided I'd rather start an earlier save.
Another accident almost led to casualties, but I described it in length in another topic. In short, a miner was trapped after digging the start of a drainage channel for a farm under an aquifer. A series of mishaps (including very wrong pressure plates) led to water quickly filling the room. After a series of futile attempts, the miner was rescued - at the last possible moment - and dropped sleeping on the same spot he washed out at, already a normal-grade Swimmer. The whole problem was lack of grates to put over the dug channel. During the whole ordeal, the mason that was doing them managed to do one run, and almost bring to his shop a stone that he brought from 5 levels below and across two fiendishly long winding corridors, that he chose over another that lay on a stockpile 7 tiles away from his shop.