I started a new fortress in 34.06 - it was my fourth one, had a couple in 31.25 and one in 34.05 which I abandoned because of a new version (and because it wasn't going well).
First facepalm was that I did start looking for water and nice underground farm plot areas too early, without making an underground storage area. The coati men killed a dog and prevented the dwarfs from getting to the stuff in the outside stockpile - until I recruited the woodcutter to the militia. He got them all soon enough.
That was the small facepalm. The larger one came when I tried to build the well. I have an aquifer, and it's possible to dig around it, as I have a mountain biome which doesn't have the aquifer, so I can easily dig to the side. As I want a well, I decided to dig a small cistern and fill it from the aquifer.
Now, usually I have dug the cisterns as rectangular multi-level rooms. A friend gave me the advice that I could dig an inverted pyramid, with slopes on all sides so that the dwarfs can even climb out of the pit, so I proceeded to do that.
Of course, without thinking about it too much, I designated the whole pit at once, from the top down. The last parts to be dug were, obviously, the ones on the topmost layer. For some reason both my miners (I had two, two picks and had had three immigrants to the fortress at this point, so 10 dwarfs) got to the process of finishing that top layer.
For some reason they both were either on the slab when it fell down or under it. *double facepalm*
Result: both miners dead, at the bottom of a pit. Apparently the cave-in removed the slope at the bottom so I couldn't get to it, and I couldn't figure out how to build ladders there.
Luckily the caravan arrived soon enough and I realized I had more than enough tetrahedrite to forge a new pick, so apart from a couple of tantrums and the loss of the skilled miners I'm again good to go.