The episode: “The Legend of Boggy Creek II: And the Legend Continues,” ep. 1006
The riff: Announced in a crotchety old man voice by Tom as the titular creature sits on a hilltop and sends a fisherman running for cover.
The explanation: This is a reference to the “Planet of the Apes” series, specifically a concept that was common in the numerous terrible sequels to the original classic starring Charlton Heston. The “Forbidden Zone” changed in meaning from movie to movie, but was generally understood to mean areas that the apes couldn’t or wouldn’t go. These included irradiated surface-world ruins of major metropolitan cities on Earth such as NYC, which had been destroyed by the wars of mankind. The Forbidden Zone was usually populated by mutants of some fashion.
Novelty factor: This is one that I pretty much always guessed at, and it turned out that I was right. Throughout the entire MST3k series, there are many, many “Planet of the Apes” references. From bomb-worshiping cultists to particular catchphrases like “A planet where apes evolved from men?!”, it is one of the series most often-referenced franchises.