Short description of error
when a light group its used in a Blender internal material in X scene it automatically lits all the materials on the others scenes
Exact steps for others to reproduce the error
2- Set the light group of the with plane with the Group (Group its the group of the sun light)
3-Go to the scene "Scene" and render
4- you should see the red plane,
the thing its that in that "Scene" there isnt a light, its being lit by the sun on the scene "SCENE-2"
5-erase the light group from the material on the plane white on the "SCENE-2"
6-Now try to render the red plane, it should render black