This is wierd. Somehow the normal of the
plane is being rotated by the clear spheres.
Must be some sort of bug. =(
The effect goes away if you remove the
normal from the plane.
Try this as a replacement...
#local cpig = pigment{checker Red*1.7 White*1.7 scale 0.05};
#local bpig = pigment{Black};
plane {y, 0 texture{pigment{bozo pigment_map{[0 cpig][1 bpig]}}}}
