I've recently had cause to go back to Slings and Arrows to have a look at the way they staged the Macbeth-within-the-Macbeth-related season of the series.
If you've never seen this show, stop now and go watch it in its entirety and then come back. A remarkable show in its own right, the fun it has with Shakespeare push it into entirely new categories.
If you've never seen the show but you're not obeying the command in the last paragraph, you'll need to know that the series is about a director who takes over as the artistic director of a Shakespeare festival when the previous director dies. That previous director's ghost (or is it just a hallucination?) returns from time to time to help (or to plague?) the new director. In the episode from which this clip is drawn, Jeffrey Tennant (the new director) goes to a local grade school to see its production of Macbeth (the grade school always puts on a version of whatever the festival is doing). As Jeffrey watches the show, he becomes more and more disturbed by the thought of having to direct the play himself—until Oliver, the previous director, shows up. Here's a compilation of all the scenes with the grade school Macbeth:
It's great stuff, particularly in the layering that gives us a director of Macbeth behaving like Macbeth when Macbeth sees the ghost of Banquo.
Links: The Show at IMDB.