Other mistake: As Charlie and Bart are starting to get up from the quick sand, the light reflector is jostled, thereby causing the light to jump.
Other mistake: When the Western fight breaks into the dance movie set, the background of the Western movie is seen, even though it was all several hundred feet away, on an outdoor set.
Suggested correction: This is entirely in keeping with the "reveal of the movie within a movie.
Of that, there's no question. I was pointing out that the sets for both movies were a distance apart...the Western backdrop should not have been seen.
Other mistake: When the gang rides into Rock Ridge, a cigar smoking man, standing in front of a door, takes a bullet through his hat, front to rear. There is no mark on the door behind him.