Quote - "It will probably work in Poser 1 thru 4 because this is just a problem
specific to the Firefly renderer...."

Almost but not quite true (to be pedantic..)
In P5, using the double-sided square (or almost anything which has double-sided polys) with the P4 render can also cause artefacts. Different artefacts, and not so noticible, but there...
They show up as thin, horizontal, randomly-spaced dark lines....

Interesting.  Because although I almost exlusively use Poser 6 now, I still have Poser 5
loaded on my machine.  And if I use the Poser 4 renderer with Poser 5 the odd little
rectangular boxes don't show up.  They only show up when I use the Poser 5 Firefly
renderer.  But then again another reason why I almost always use Poser 6 anymore
and just don't have to worry about the Poser 5 bugs.  In fact about the only time
I fire up Poser 5 is when a Poser 5 question like this pops up.