I have a double facepalm incident, which is mostly my fault.
In my latest (and longest-running to date) fort, I accidentally allowed using bars and/or blocks to construct walls and floors, instead of my usual "blocks only" personal rule. This normally wouldn't be a major problem, except I was mass designating a floor to be built above the trading post for a second-floor tavern I had planned out so there was a lot of floor to craft. I kept getting notices that my metalcrafter had no silver with which to craft, in a last-second effort to see if I could afford the giant grizzly bear the elves brought with them. That's when I scrolled up one z level and saw the entire tavern floor was made of iron and silver despite a ridiculous quantum pile of stone blocks.
Annoyed by the setback, I marked the entire section of metal floor to be deconstructed. I forgot that dwarves have no sense of physics or self-preservation.
Almost immediately, they proceeded to dismantle the tiles in such a way that three dwarves fell on top of the elves in the trading post, and a total of 8 people were knocked unconscious. Only 3 dwarves were injured. Somehow the traders didn't even get negative thoughts from being knocked out and covered in peat dust.
Face palm number 1.
Then, I realized that the most significantly injured dwarf (who landed on his right side, mangling his right arm and leg, damaging his right lung and somehow severing an artery on an iron bar) was the chief medical dwarf!
So I had to quickly assign some ad lib doctors, craft extra splints, and panic slaughter a goose to make soap. By the end his entire body needed to be splinted and bandaged and sewn. It's 2 years later in-game and he's still permanently winded from the lung damage, though I think he's finally getting a little bit faster again. And the other dwarves that got knocked out still have negative flashbacks to the time they got knocked out during a cave-in. Two of them got knocked out a SECOND time during a botched tree-felling incident, along with a third previously-unscathed dwarf.
Face-palm number 2. This one was unavoidable but the fact that it happened a second time in the same year feels ridiculous.
All because I forgot to forbid floors from being built with metal bars. I'm still mad at myself about it. The second time I tried to dismantle that floor, I designated one row at a time so nobody could be a dumbass again.