Cheers had one of my favorite fourth wall breaking jokes in it. After Shelly Long left there's an episode where they all go to a drive in for a Godzilla movie marathon.

One of the characters (I think Cliff) explains to Woody that one of the actors from the Godzilla movies quit and left the franchise. Woody responds with something like "Why would an actor quit in the middle of a succesful franchise? That's just crazy!"

It took me a second to realize they were making a joke about Shelly Long leaving the show but when I did I couldn't stop laughing.