-Ceiling one block thick, preferably spanning several chunks. Made of solid, opaque blocks.
-Fly away so the chunks are unloaded. Simply exiting the world does not trigger the glitch.
-Come back and enter the room. any part of the room near you turns as bright as day.
-Unloading the chunk again or exiting the world brings the shadow back, until you get near it again.

Note that the hole in the ceiling was put there only after the glitch occurred. It is only there to show the thickness of the ceiling.