I was having an issue with G4Tessellated slids randomly deleting facets, resulting in broken geometries.  I have traced it down to an error in G4VFacet::operator ==.  Thie issue is that, as written, the loop for a 3-sided facet runs from 0-3 (inclusive) due to a misplaced autoincrement.  Here is the original and the fix.

ORIGINAL -- broken

///////////////////////////////////////////////////////////////////////////////
//
G4bool G4VFacet::operator== (const G4VFacet &right) const
{
  G4double tolerance = kCarTolerance*kCarTolerance/4.0;
  if (nVertices != right.GetNumberOfVertices())
    { return false; }
  else if ((circumcentre-right.GetCircumcentre()).mag2() > tolerance)
    { return false; }
  else if (std::fabs((right.GetSurfaceNormal()).dot(surfaceNormal)) < 0.9999999999)
    { return false; }

  G4bool coincident  = true;
  size_t i           = 0;
  do
  {
    coincident = false;
    size_t j   = 0;
    do
    {
      coincident = (GetVertex(i)-right.GetVertex(j)).mag2() < tolerance;
    } while (!coincident && j++ < nVertices);
  } while (coincident && i++ < nVertices);
  
  return coincident;
}

CORRECTED:

///////////////////////////////////////////////////////////////////////////////
//
G4bool G4VFacet::operator== (const G4VFacet &right) const
{
  G4double tolerance = kCarTolerance*kCarTolerance/4.0;
  if (nVertices != right.GetNumberOfVertices())
    { return false; }
  else if ((circumcentre-right.GetCircumcentre()).mag2() > tolerance)
    { return false; }
  else if (std::fabs((right.GetSurfaceNormal()).dot(surfaceNormal)) < 0.9999999999)
    { return false; }

  G4bool coincident  = true;
  size_t i           = 0;
  do
  {
    coincident = false;
    size_t j   = 0;
    do
    {
      coincident = (GetVertex(i)-right.GetVertex(j)).mag2() < tolerance;
    } while (!coincident && ++j < nVertices);
  } while (coincident && ++i < nVertices);
  
  return coincident;
}