I've run across this before and have never found the answer.

Here is a screen capture of a scene called "Gymnasium." Its a simple basketball court (I think a freebee... been so long since I got it I forget where).

Note the far wall... it is not textured, being colored only by the difuse color shader in the material room.

 Here is the same scene rendered. Note the dark squares that have appeared out of nowhere.

This is not dependent on quality settings, normals forward, of hiding backfacing pixels. Does anyone know what it IS that causes this, and more importantly, how to fix it?

Thats double sided polygons being a pain.
You can try adding a little bump to the surface to help poser sort out the two sides.
I did a sample pic using the three prop squares in another thread recently on this same problem .

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3530736&ebot_calc_page#message_3530736

 Right you are! Normals forward and 1% bump fixed it right up!

Many thanks!

You are welcome . Its been a problem for a long time , keeps coming up with new users and new modellers  :)

and new versions of Poser... P8 + SR1 re-introduced this in a scene that worked pre install of SR1.
Noise injection worked.