Here is some info of the scene. My version of LW is 9.5. I got a scene with a tree, imported from xfrog, with its maps for color, transparency and bump. They are applied on the surface editor, not in the node editor. The light is Spot with raytrace shadows. No use of radiosity. No animation on the scene (yet), by the way... i´m just testing Hd instance.
1-Scene screenshot: http://farm3.static.flickr.com/2561/3668960444_9aca519e79_b.jpg (http://farm3.static.flickr.com/2561/3668960444_9aca519e79_b.jpg)
2-Render 1 (works fine): http://farm4.static.flickr.com/3383/3668150643_87487bfd7a_o.jpg (http://farm4.static.flickr.com/3383/3668150643_87487bfd7a_o.jpg)
3-Render 2 (BAD one): http://farm4.static.flickr.com/3563/3668957040_3184b1079b_o.jpg (http://farm4.static.flickr.com/3563/3668957040_3184b1079b_o.jpg)
The 2nd and 3rd images were rendered with same settings (shown in 1-), so its a kind of 'bad mood', i guess..