Wait for the part of the movie where the the star has just arrived in the far far future, after the volcanic rock wears away around him but before he stops the machine. There is a part where you can see a stone wall being built behind him, brick by brick. If you pause and move one frame at a time as the wall is being built, there is a single frame where they forgot to put in the background. The bricks, sky, and everything beyond is replaced by a blank paper on which you can make out the words "The Back Frame."