POV-Ray

Overlapping partially transparent objects can result in speckled shadows.
Rays shouldn’t leak through the coincident areas, they should return one
of the two textures. This was correct in 3.6.

#declare testmat = material { texture {
      pigment {color <1,0,0> transmit 0.5}
}};

camera { location  <1.0, 2.0, -4.0>
   direction 1.5*z
   right     x*image_width/image_height
   look_at   <1.0, -0.2,  0.0>
   angle 30 }

light_source {<-10, 10, -5> color rgb <1, 1, 1>}

plane {y,-1 pigment{rgb <1,1,1>}}

union {
   sphere {<0.0,0,0>,0.5 material{testmat}}
   sphere {<0.1,0,0>,0.5 material{testmat}}
   sphere {<0.2,0,0>,0.5 material{testmat}}
   sphere {<0.3,0,0>,0.5 material{testmat}}
   sphere {<0.4,0,0>,0.5 material{testmat}}
   sphere {<0.5,0,0>,0.5 material{testmat}}
}