I have just discovered the second cousin of all annoying bugs.
Of course, this comes with the caveat that it would not have occurred had I not been screwing around like a looney with the raw files.
Now, the particulars: as is often the case with me, I began screwing around in the raws, looking to spice the vanilla game up a little. To this end a created a new species on a whim; a race of flying hawk men type critters. Actually, I'd initially intended them to be harpies, but apparently an all female race cannot actually expand on the civ map for some reason. 
Anyway, my general wont when it comes to DF is to build up a magnificent fortress, fill it to the brim with goodies, and then, through blatant mismanagement, run it into the ground so I can loot the hell out of it later in Adventure mode. Wanting to do so with one of my flying types, I selected a location as one of the bird-man civilizations and off I went. Only to discover, horror of horrors, that trying to do anything (at least above ground) leads to an endless string of "(Miner) cancels dig stone: dangerous terrain," etc., etc.
It took me a minute or two to figure out the problem; apparently having nothing beneath your feet counts as dangerous terrain whether you can fly or not, and any critter that can fly will fly just for the hell of it. Hmmm... wonder if hostile flying units are so beset upon? Ah well, not sure if I wanna do anything about it, though.
Being able to fly in adventure mode is just so handy.
