Is this bug or my mistake?
This is the scene hellbox.blend (96.5 KB)
The simple box mesh :o
Try to switch on Rendered view
Moved from “General Forums > Blender and CG Discussions” to “Support > Lighting and Rendering”
Looks like the material got unassigned through some trickery.
add a material and remove it and it should work.
What were you doing? If you remember how you made it do that, file a bug report.