Diolma has it dead on, Walter. Firefly does not render single sided planes very well in certain circumstances, and also does not handle places where two flat planes slightly overlap as well. Maya had the same issue with a couple of rendering engines it was using a generation back or so. The secret is to take a math node, set it to Subtract, change the first value to 0.05, and plug it into the displacement channel, which is then set at something like 0.01 or so. This will usually make the issue go away. Hope this helps :)

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)