Today I was minding my own business in my current 5-year mostly-aboveground fort, when suddenly:
"A VILE FORCE OF DARKNESS HAS ARRIVED!"
Now, I'm somewhat worried as I count 4 squads of gobbos, 2 led by a Master Swordmaster/Crossbowmaster (I'm most worried about the Crossbowmaster) along the edges of the map, ready to charge in the midst of my fortress. So I zoom in to my central keep/mess hall to lock everyone inside. Then I see that an entire squad of gobbos is already inside of my super-duper-well-guarded keep/mess hall. Past my reserve guards and markedswarves right next door in their Barracks. Even past my squad of Champion Wrestlers right outside. And nobody has even made so much as a peep about being ambushed or even cancelling jobs due to the gobbos. Unless my breed of goblins has learned the ancient art of gobojitsu, and snuck through 50 working dwarves into the heart of my fortress, i'd call this a bug.
I should probably also note that my last batch of migrants appeared from nowhere seemingly as well. Except that they teleported through a square just outside my fortress, one at a time. From total nowhere. I remember reading something about this problem before, and it possibly being caused by oddly proportioned maps or fortress sites. Just to add to the information about this problem, I am using a completely square site. My fortress is just south of the center, so that cannot be the cause. The first occurance of this on my map was just about 1 in-game month ago when that migrant wave arrived.
This is in version 40d. I'm going to back up this save in case Toady or anyone else wishes to use it. Post in this thread or PM me if you would like the file.