I suspect Dr. Geep is right (as usual). The hallway you are using may have been made for Poser 4/PP, which doesn't handle backwards polys the way P5 and P6 do.

If you're using Poser 6, try clicking "normals forward" for the floor. (It's in the Material Room.)

If you're using P5, try adding a small amount of displacement to the floor.