This might not be a bug because I didn't see exactly what happened.
I was constructing my Tower Of Power: a massive stone tower, currently about 6 z-levels tall. Next door is a smaller tower which is only accessible via a bridge from the first tower (on the 3rd floor). There are no entrances on the ground floor.
Somehow, I ended up with three dead mountain goats and a dead mule on the TOP level of my 2nd tower. Note that the ONLY way to get into this tower is via a bridge from the first tower. The first tower has a single entrance on the ground floor, which is in turn only accessible through a narrow passageway filled with about 8 traps.
How the heck did a bunch of dead animals appear on the top of my tower?! Is it possible that one of my dwarves just decided to carry them up there for some reason? There's not a butcher workshop or anything near there...
Unfortunately, the game crashed shortly after I noticed the dead animals appearing... The tower was fun while it lasted, though
The base was 13x13 tiles, so I was getting very close to having used 1,000 stone (if you count the sister tower also).