Ah, the TV crossover - where characters from one show make a special appearance on another. You expect to see that in related productions like the Chicagos, the FBIs and the Law & Orders. But when the programs are completely unrelated and on different networks, they make fans smile - while others may not realize they're even going on at all. 

The article below lists the instance that I immediately thought of when I saw the headline. It is one of the single greatest meta moments in TV history, before that expression even existed. It involved the TV show "Community" on NBC and "Cougar Town," which at the time was on ABC. 

The character Abed long rhapsodized over how much he loved Courtney Cox and Cougar Town and in one episode, he talked about going to L.A. on his summer vacation and winding up as an extra in the show. But when he got nervous and "pooped my pants," everything went of the rails. 

On that week's Cougar Town, you saw it happen, as the character was seated behind the main actors in a scene and suddenly got up and ran away, crashing his chair behind him. There was no explanation. If you didn't watch both shows, you never would have known what that was about. But if you were in on the secret, it was incredible. 

You can see those scenes and more in the story below.

The Forgotten TV Crossover Episodes Only Geeks Will Remember