I don't think there is anyway around it, but a super quick fix is:

#include "colors.inc"

/* Prints a 'L' in a white pannel */
difference {
 // The panel
 box {<-1,-1,0.5>, <17,12,2>}
 // The 'L'
 union {
  box {<0,0,0>, <1,9,1>}
  box {<1,0,0>, <6,1,1>}  //fix the coincidence surface

  pigment { color Red } // To see the L print
 }
 pigment { color White }
}

light_source {
 <8.5,5,-40>
 color White
 shadowless
}

// The camera looks into the 'L''s corner
camera {
 location <0.5,0.5,-2.5>
 direction z
 right x/3
 up y/3
}

Just don't have the objects in the union overlap the same space and you won't
have a problem.  The docs say that it is a numerical problem and makes no
distinction as to whether or not colors are the same.  I get the impression that
the problem is in POV finding surfaces and two surfaces occupying the same space
causes the background to show through, regardless of the color.

Deneux Johann wrote:

>     ...
> I look for the nearest intersection..."
> Here is the problem: both intersections are at the same location. So, will
> the color be the first or the second box color ?
> But there should'nt be any problem, as both colors are same. Where do the
> white points come from ? This could eventually be the panel, if its bottom
> had been at Z=1. But it's not the case.
>

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