I have a bunch of facepalms, but I just now got one worth registering for, so you get a funpack!
First of all, I'm still a newb. So I make newb mistakes. I'm learning slowly. But that's why my facepalms tend to come in packs.
One fort, for instance, was a series of facepalms.
I hit the caverns pretty early, due to the topography of the area. 'Cool,' I thought. 'Mud! I don't have to build a flood setup!' So I happily started farming. And then I get spammed with "Urist McDorf cancels job: interrupted by troll." I forgot about cave beasties. Just as I'm wondering if I have enough coffins, I get a message about one of my dogs has been struck down. 'That's odd,' I think. 'They're all on the surface...' So I go looking for the spot. Find the dog corpse, but the troll is nowhere to be found. Then I check the unit list. Troll's not on it. Apparently, the troll ran up from the caves, straight up through my fortress, and when it hit the surface, it just kept running. Didn't touch a single dorf.
Despite that object lesson, I still didn't take the appropriate measures, just leaving the caverns open. I got distracted by something else, I believe I was trying to scramble to get the trade depot up and running. And then I get spammed with more job cancellation messages. This time it's a giant toad. 'Pft, how much trouble can a toad cause?' Aaaaaaaaand then I get the 'Urist McDorf has been struck down!' messages. By the time I get to the scene, one of my master miners and a hauler are dead, and another hauler's bleeding to death, and the damn toad has a name and a title. It could have been the death of my fort, except the killy times happened in the proto-corridor that was to become my trade road, and I'd had the foresight to put a door there. I waited for the miasma to clear, put a row of cage traps down, and let the toad out. Caught him as soon as he hopped out. And then I floored off that damn cavern. (And put walls in when an FB spawned down there.)
Later, I was working on another project. I'd read a lot about these awesome pump-powered waterfalls that work like happiness generators. So I looked up the wiki, studied the standard plans, and thought, well, that doesn't look too hard! So I started digging out part of the assembly layers. Dining room layer was fine, no problem. The reservoir under the dining room, that was easy! And then I started digging up, to make the top of the pump layer.
And I forgot that I had dodged around the aquifer to get to the dining room layer in the first place.
To make things even better, I had also started my first attempt to make a well and a reservoir filled from the river for it. Forgot a little thing called pressure. Blew off my well, and flooded my hospital. So, the hospital was a mess, and I'd just lost a fully smoothed and engraved dining room, with several masterwork engravings. And then I ended up losing a couple of dorfs to drowning. The well flood was blocked off with a door, and the aquifer was a slow flood, so there *shouldn't* have been an issue evacuating. But the idiots kept running back to the dining room to clean, even after I forbid it. Then they ran around in the halls near the dining room, which were also flooding. I'd walled off all the corridors but one space, to contain the flood. I was waiting for the rest of the dorfs to evacuate so I could put the one last wall in. And they just... stood there. In water up to their waists that was steadily rising. With the safety of the halls not fifty feet away. I left them to their moist fate when the water got too close to the main hall and walled them in. Fortunately, they were new arrivals, so they didn't spark a tantrum spiral.
*multiple, multiple facepalms*
I still haven't reloaded that save, I don't want to clean up the mess. >>
On another fort, things were going nicely, right up until the macaques showed up. They got through my cage-trapped lined entrance and got some valuable stuff. So, I mobilized my three squads of half-trained militia and sent them to hunt the buggers down. They got two of them, but the third was leading them a merry chase around the map. They finally cornered it at the river and emerged victorious. And then two of my recruits fell into the river and drowned.
At least they didn't have anything important? *facepalm*
And this one happened just a little while ago, and was so stupid I had to share it. I knew I was in trouble as soon as I saw my embark name. Uristnom. Nothing good can come out of a name like that. But I embarked, and started carving out a rudimentary base, just a couple of workshops, beds, and stockpiles, and the early stages of my farm.
"Urist McWoodcutter cancels job: interrupted by alligator."
... What?
And sure enough, the doom predicted by the name came true. The alligator mauled six of my seven to death. The seventh tantrumed, and then the alligator mauled him.
"Your fortress has crumbled to it's end."
No, really? *facepalm*