Killed a FB made of water, dwarves unable to clean the remains (pools of water). Some parts were solid like head, which I had designated to dumping, but deactivated the dump site when they were still hauling, and these turned to water as well as they dropped them and now are uncleanable. Dwarves periodically try to clean them, as in they go stand on tile and clean, but the puddle remains. There is no job cancellation error. Both "original" puddles and the solid-then-liquid ones are apparently permanent fixtures now. They've been on my fort for about 2 dwarf years now. (Happened in version 0.40.8, after updating to .9 still same thing.)

Gray puddle near the Dwarven atom smasher, close to the dumping dwarves (F2). Other puddles at F4 (first cavern) also uncleanable. There is probalbly couple more somewhere in the stairwell. I also noticed that the puddles at f4 also have goblin blood on same tile, which aren't being cleaned. There has been no goblins in that area so I suspect these got on top of the water from dwarf boots. After killing some other FB at the same spot, I witnessed a dwarf cleaning the mess. He did clean the fluids of the new FB, but the water and goblin blood remained on the tiles.

EDIT: Dead trees (after their upper parts got burned by raging fire from FB breath) cause deadly cave-ins if chopped down.