There are many movies, particularly about Hollywood, that have a flim inside of a film. (a movie inside a movie) A recent example is D'Niro's 'What Just Happened'. The film inside of that film starred Sean Penn.

These films inside a film are the least important by far,

unless you include cartoons like 'The Little Hawk that Couldn't'
IT taught kids that it is OK to fail, in fact good things can result from your failure.