In the episode when Kramer reverses his peephole:

When Newman knocks on Kramer's door and looks through the peephole, you can see the inside of Kramer's apartment. Remember this background. When Kramer opens the door, the background of his apartment is completely different (by completely I mean different everything) from the background that Newman sees.

One example: The back wall in the peephole background is wooden (or wood colored). The back wall when Kramer opens the door is a light green drywall (or something similar).