I've had that problem when I made a bunch of bushes using transparency mappes on a terrain. I did a video, so I saw that when a bunch of bushes were lined up in front of the camera, there were the black cants, when the camera reached the top of the hill, looking to the sky, not many bushes were lined up, and the black cants were not present. What renderer are you using? I saw this problem in ART, but I didn't try the scanline, which might change things. I think it has something to do with the maximum reflection value for the rays in the ART raytracer (if it is the renderer). So see if increasing the value to some huge number helps (if you plan to not use reflections that will see reflections, or you'll sit there forever as it caluculates the relflection) or try changing the renderer.