Wasn't it J?rg 'Yadgar' Bleimann who wrote:
>Hi tracers!
>
>After doing some other things for a while, I returned to my great
>POVghanistan project, beginning to play around with Mark-Hendrik
>Bremer's suggestions for mud brick wall isosurfaces...
>
>But when I tried to use conventional CSG to cut a door and windows in
>the main building block (the subtracted objects being isosurfaces
>themselves), I got those strange results you see in the first picture.
>Obviously, the rounded_box part of the house's back wall has
>disappeared, while the irregular_bricks part still remains... bizarre!

It sounds like you've not read the bit about max_trace in the docs:

  Isosurfaces can be used in CSG shapes since they are solid finite 
  objects - if not finite by themselves, they are through the cross 
  section with the container. 

  By default POV-Ray searches only for the first surface which the ray 
  intersects. But when using an isosurface in CSG operations, the other 
  surfaces must also be found. Therefore, the keyword max_trace must be 
  added to the isosurface statement. It must be followed by an integer 
  value. To check for all surfaces, use the keyword all_intersections 
  instead. 

  With all_intersections POV-Ray keeps looking until all surfaces are 
  found. With a max_trace it only checks until that number is reached.

-- 
Mike Williams
Gentleman of Leisure

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