From Muppet Wiki

The Stage Door of the Muppet Theater was seen in episodes 204 and 316 of The Muppet Show, and The Muppets Go to the Movies. One of the Muppet Show albums also featured audio set in this area.

During the fifth season of The Muppet Show, each guest star made their entrance through this door to meet Pops in reception. Other notable uses of the stage door have included Statler and Waldorf hiding from Manny Kaye in episode 316, Fozzie returning from group therapy in episode 311, and Gonzo's exit after his motorcycle fails to perform in episode 315.