taking a breather on this one -- I've been working on it pretty heavily for three days now. If it's a bug, it's only partially duplicated through variations. I'm leaning towards a computational limitation (call it a bug if ya want, though, my being a stickler don't mean yu gotta be). Meaning: it's out of the range of the rendering engine. Might not be, but that's what I'm leaning towards. It's ray based, proximity involved, shadow independent, and object independent. Facing of polys does affect it, but only slightly. It occurs mostly, though not always, when you have a situation of multiple shadowed areas in a rayed scene involving transparency, reflection, and refraction. To effectively track it down exactly, your entire set up: lights, materials, and render settings. I had my blinders on as well - Ajax is right. Based on the method by which this stuff works, the sum of refraction and reflection needs to equal 1 in the Poser_Surface.

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)