"Last note from Radiance :

"the only remainins 'shading artifacts' is the intersections on low poly geometry with direct sunlight,
which is'nt an artifact, it's just something that's natural, there is no physically correct solution to it." "

I don't read the whole thread, but if I understand right ... so far I know it's a known general render technic problem. I can produce this kind of error per Vray too.