OK.
Something very simple to do:
1. take the woodbox.pov scene;
2. add a radiosity block;
3. uncomment the area lights;
4. start render and stop when some part of the image is ready;
5. add +c;
6. restart render and watch if problem occurs at the boundary between
already rendered image and newly parsed one;
7. if not, stop render and repeat process until problem appears.
Thomas
