I'm going to make an absolutely uninformed observation here, because math isn't my strong suite; the leakage/interference patterns on Miss Nancy's post above and most of the light leakage on BB's first experiment are what I would call random, or the visual equivalent of 'white noise' (it won't be random, it's calculated by an algorithm, but if math is not my strong suite, chaos theory is worse). However, BB's last image on scatter shows a very regular pattern of shadow artifacts along the floor/back wall seam (and it looks like a lesser version along the back wall/ceiling seam). I went back and had a look at BB's earlier images, and some of those are regular as well. As BB's geometry is 6 single faces (no subdivision) then it can only be caused by the render algorithm...right? I have no idea if that is useful or not, but just posting an observation.