I don't have too many big mistakes, since I'm normally fairly cautious. Most of them were on my first fort
Whipcaught. Let's see:
I had an overhang of rock, because there was exposed platinum on the surface as one of the first metals I had found, and I had mined it out. But there was still a floor left above, and it provided an area that attackers could hide under outside my walls, so I decided to remove it. What's the easiest way to remove it? Mine out it's support and it'll come crashing down to the ground. So I did, and it collapsed, and hit the ground.. and kept going, down through at least 4 or 5 levels of my fortress, killing a few dwarves and smashing a few artifacts among other things, and leaving me with a large multiple story hole through my fortress where the floors used to be.
Another one, is when I was trying to get crocodiles out of the cave river and capture them. I had a nice idea of making a set of grates by the edge of the river, and then I'd dig into the side of the underground river, the water would pour out, fall down through the grates a couple of levels, and get pumped out to the surface, while the crocodiles would walk across and into the traps. And I had it all set up and controlled by levers from a distance, and I channeled out the tile to the river from above. When I realized, that I can't make a 3-wide entrance out of doors. I had known this but had somehow overlooked it in my construction. So when I tore down the center wall, it was all the was holding the center door in place, which got deconstructed, and let all the water flow through with no way to stop it.
Well, I thought, ok, that's a problem, but it's not that big of one, because it'll just get pumped back out to the surface. Which is when I noticed the second mistake that I had made, I had been constructing the pump tower from below the grates to the surface, and so I was leaving sealing off the bottom of that pit as the last thing to do (because workers needed to get in there). Well, I had apparently forgotten about that, so instead of just dropping down to the bottom of the pit and getting pumped to the surface, it was flowing out into the rest of my fortress. Fortunately it was my tree farm, which was already mostly sealed off, and I was able to get any vertical passages out of it sealed off before the water had time to spread very far. Although it did flood my mechanics level that I needed to get power to additional pump towers. But I did eventually get a couple more pump towers built, and reclaimed the flooded areas, and got a working door built to seal off the cave river breach.
Other then that, my artificial waterfall lake was probably a bit of a mistake. I built an underground multi-level lake, and filled it, and had pumps from the bottom of it back to a few levels above it. So there would be constant waterfalls around it, and I built a floor out to the center of it from the sides where there was a central well. But I underestimated how fast pumps move water around, and how willing dwarves are to cut corners, and how easily a bit of water can apparently push a dwarf off of a narrow slippery floor.
So instead of some gentle waterfalls into a lake, it ended up with massive torrents of water pouring down into a giant whirlpool, the constant flows of which pulled anything that fell into it down to the center of the bottom of the pool. Which ended up being coated with a layer of bones that slowly built up over time from whenever anyone fell in. I eventually remembered to mark all the water areas as restricted, and that helped cut down on fatalities a bit.