Just started a new fortress, and decided to make a glass industry hall. My volcano was about 15 z-levels higher than the fortress, and I didn't want to risk my miners by breaching it from the side, so I decided to make a long drop-shaft to deliver the magma to the furnaces. I ordered the miners to channel down from the top, sealing them in and forcing them to finish the project; then I switched to stairs for quicker completion... but one of the miners was my expedition leader, and there was a liaison on the map. So halfway down he decides to conduct meeting with him - he was standing on the side of the shaft. The other miner sealed in went on a long, long break. I ordered the remaining miner to dig out a way out, and then I found out why they didn't continue: the very last level of stairs was not upwards, but downwards.
Facepalm one.
The last remaining miner keeps digging, but does so very slowly and I realise why only a couple of minutes in: the tunnel was 2 tiles wide, as I expected the sealed-in miners to continue it, and forgot to redesignate it for quicker digging. Also, the miner didn't have his Masonry turned off (every dwarf had masonry on, there was a lot of building going on), so he constantly left the tunnel to build another floor.
Facepalm two.
The miner was just a few tiles too slow, and both of the sealed-in miners died.
Facepalm three.
Re-try of the same location (hate quick losses to stupidity, so restarted): I order a shaft to be dug on the same spot, but order several z-levels for channeling, forgetting the stairs for some reason. Cue the dwarves channeling out the tiles they were standing on, over a few levels of already open space... critically injured miners happen.
Fourth facepalm.
Another facepalm edited in:
In the previous fort, I've built five Goblin Grinder design siege lifters. They all worked fine... except for the fact that I put green glass disks in them. The poor goblins' frantic dodging of the disks and getting their armor dinged spawned 300-400 combat pages a second. They kept falling down from exhaustion but all remained safe and sound. *Facepalm*