I checked and the square primitive is a quite nasty thing, made up by two squares coincident in space and with opposing normals. Not surprised that bad things happen; probably Lux works because it does not detect interactions with the back plane (I know that when you use it to render fluids in bottles you have to very slightly scale the mesh representing the fluid in order to big Lux a reliable glass->fluid interface).

Bye!!!

GIMP 2.7.4, Inkscape 0.48, Genetica 3.6 Basic, FilterForge 3 Professional, Blender 2.61, SketchUp 8, PoserPro 2012, Vue 10 Infinite, World Machine 2.3, GeoControl 2