Light leaks and shadow leaks are the result of faulty calculations because of the way the polygons on the floor match up or rather, do not match up with the wall polygons. The explanation in 3D Lighting, Arnold Gallardo is so jargon ridden and unclear, I am not sure he understood it. But the figures in the book show it right enough. The way to reduce it, seems to be, to have high poly props, quads rather than triangles - especially no acute triangles - and for the polys on the two surfaces to be done in parallel. Two planes that meet at right angles and the plane cutting across the open areas of the other's polygons? Render engines seem to have a problem intrepreting light and shadow over the surface of the transsected polygons - partially ignoring the effect of the prependicular plane on the light on polygons who live on either side of the perpendicular plane. But, as I said, the explanation is clear as mud. The book focuses on TS, Max and LW - so if those render engines have this problem, it would be unrealistic to expect Poser or Bryce to not share in it.