Are the mesh (Wall and floor) one object? maybe the floor is lower than the walls just slightly. Also you can try to add a plane or box as a test floor and let it overlap the walls just slightly and see if that stops it, just to see if its the model or Poser. Its a bit difficult to see what is going on. Maybe if you make a close up render of the door and wall in the background and do a wireframe view as well. Just do so you show the wireframe view (Ctrl+3, or third option in document display style) in the viewport and take a screenshot using printscreen.