Wasn't it Myself who wrote:

> A hole appears in the blob when the surface of the transparent sphere
> is close to the surface of the blob, and between the camera and the 
> blob.

I did a bit more thinking about the conditions under which the effect
appears, and came up with the idea that perhaps the algorithm that finds
where a ray intersects the blob surface is failing when the ray starts
very close to the surface. [The logic being that the first traced ray
stops at the invisible surface, then a second ray might start from that
invisible surface going in the same direction.]

This hypothesis leads to the prediction that it might be possible to
observe the same effect by reflecting the rays off a mirror that's very
close to the surface. In this case a ray restarts at the surface of the
mirror, and the hypothesis suggests that there would be a region where
the ray would sneak through the front surface and hit the interior.

This does, in fact, turn out to be the case. The interior texture of
this red blob is bright green, and a sliver of green can be seen where
the blob touches the mirror.

global_settings {
  max_trace_level 50
  assumed_gamma 1
  }

background {rgb z*0.3}

camera { location <0,1.5,-3> look_at <0,-0.6,0> angle 40 }
light_source {<-100,200,-300>, 1 }

blob { threshold 0.1 sphere { <0,0,0>,1.2 ,1 } 
        pigment { rgb x }
        interior_texture{pigment {rgb y*4}}
}

plane {y,-0.5
 finish {reflection 1}
 pigment {rgb 0}
}

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