The movie within the movie.

[Pastor of Muppets]

There's a point in this movie where the teen boys are all hanging out in an RV watching some movie.  You only see a few seconds of it; a scene where an ape grabs some sort of egg and then flames start shooting out his ass.

About 10 years ago James Rolfe (also known as the Angry Video Game Nerd) became obsessed with finding that movie.  He found it, and did a video reviewing the movie and that scene especially.

Watch here.

Essentially, it's part of a series of really weird Italian caveman slapstick movies.  The scene is from a movie called Grunt.