I just wrote up a long (hopefully witty) set of stories about my experiences in 5 fortresses in .21 & .22 and just had it all wiped out by my hotel's wi-fi login screen while trying to post. *face palm*
To summarize, things that caused a face palm:
1.1) Animals will die without being put in pasture despite copious vegetation because they are too stupid to feed themselves without being told to.
1.2) Dwarves you decide to deliberately let starve in hopes of using their work to lay the ground for your "real" embark team don't die; they just work slower while useless migrants pile up.
1.3) Letting 60-odd dwarves die by abandoning a fortress creates 2-3 years of hauling labors just to put everything back together. Mostly thanks to clothes.
1.4) Letting 60-odd dwarves die without creating a bunch of coffins first leaves a place a wee bit haunted.
1.5) Dwarves too busy to pay attention to a room with nest boxes for 10 turkeys will fuel the creation of a blood-spattered turkeydrome filled with hundreds of murderous baby birds that will come rampaging in a rage out of their pasture to hound down their chosen victim and who have to all be herded back in.
(On the bright side, I did find the bloody corpse of a honey badger in the doorway to the room at one point. I promptly named him "Link.")
2.1) While turkeys crank out an impressive number of eggs, eggs seemingly cannot substitute for bringing plump helmets and farming. I tried to correct this enthusiastic oversight while all my dwarves were starving, but they were too busy scrounging for vermin to farm. I found out 3 years later that the reason they were starving is because eggs (unlike meat) have to be cooked.
2.2) Then I found out that the whole reason I did this -- to avoid irrigating underground soil in 2010 -- is no longer an issue.
3.1) I carved a too long magma pipe from the topmost level of a volcano to your sand/clay furnaces 2/3 of the way across the map and found out it would have taken 5-7 years to fill if ever thanks to magma evaporation.
3.2) I made it worse by accidentally creating a 2 level reservoir through stairs underneath the tube.
4) I embarked on a perfect site with nearly perfect dwarves after 4 hours of world gen and searching (sand, clay, shallow & deep metals, only 3 Z-levels of hills to keep track of, a volcano, no aquifer, and a waterfall) only to find out that the bottom of the volcano has holes in it that emptied into hell as I watched all that effort drain away as the surviving demons bided their time, as the joke goes, to tell me that the dog wasn't that shaggy.
5.1) I dug a channel from the river all the way to the first level of the caverns so that I could have an opulent dining room / meeting hall with a waterfall in it only to find 1 year later that the growth of cave grasses had completely choked off the stairs I used to drain the water and left the bottom of my fortress flooded with no way to fix the bottom of the pipe 7 Z-levels below.
5.2) Then I found out the next morning that Toady One fixed the bug that caused grasses to clog stairs and released a fix the Friday before I had started the fort.