I have quite a few facepalms since I just starting playing less than a month ago.
Fort One I flooded partly since I did not know the joy of pull the lever to close the flood gate. I then was putting a well down in my magma workshops for the hell of it and did not know the the water would work like real water which would cause it to flood half of my supply rooms and wash away all my ores.
Fort two, Cyclopes shows up and I think now big deal since I have ten dwarves in full iron and a few traps in front of my fort. So I thought lets have my guys beat him up after he goes through the traps, so I put them right behind the traps. I then waited, and waited, Then I found out that he got into my base since I dug up so many walls to get all that iron. The cyclopes only killed one dwarf, the dwarf that EVERYONE else likes. You know what happens from there.
Fort three my first above ground , a small face palm since I almost forgot to make anything to trade with the first merchants and I almost forgot to make a trade depot. Next I tried to move a rive to come in and out of the fort for my hospitable and fishingdwarves, I channeled out one z level to low which cause it to fill the lower z level and then began to fill the one above it, I was lucky enough to see that it was doing this pretty earlier one so I had my guys make a wall to block off the river which two of my dwaves were on the wrong side of this wall. These two guys tried to swim up but ended up drowning, I lost my broker and some random dwarf. Then a FB shows up in one of the caverns, so I walled off the cavern since I had not military yet. He goes away and then shows make up in my stair case since they did not finish the wall ever.
Current fort another above ground fort, Well the only thing I face palmed about yet is the stupid buzzards will never stop bothering me.