It is quite common for characters within one work of fiction to reference another. If this show takes place in our Universe then it is to be expected that the characters would know about any other show that exists in the real world. The only real show that by default doesn't exist within a story is the one you're watching right now (and there are some exceptions to even this).
On the other hand, there is no reason why the author is required to stick to real media. Sometimes, an author will make references to a work that was never written in the real world, even if it's completely real for their characters. Some other times, the author will use a non-existent book to mock something. Or use it as a Take That! for a film, or just as a parody. Characters in series may listen to platinum albums that were never recorded, watch movies never filmed, or play videogames never programmed.
This is not necessarily just a throwaway reference either; sometimes the whole point of the main work is based upon this Fictional Media, for example a review of a non-existent book, or a trailer for a film that no one had any intention to make. Common types of Fictional Media include:
- Different World, Different Movies
- Fictional Painting
- Fictional Video Game
- Game Within a Game
- My Little Panzer
- Niche Network
- Real Trailer, Fake Movie
- Ridiculous Future Sequelisation
- Show Within a Show (arguably the Super-Trope)
Examples that don't fit the categories above:
- Mark Danielewski's House of Leaves is a complicated example: a book that exists about a dissertation that does not exist, about a film that does not exist, about a house that does not exist. It's also implied that the book itself occurs in our reality, which would make the entire thing prophetic, by definition, if it were true. Topping it off? The book is filled with hundreds of footnotes, referencing a lot of things that do exist, but even more that don't.
- Star Wars: Shadows of the Empire was described as "The movie project without a movie". They produced a novel, comic, game, action figures, soundtrack CD and a bunch of other merchandising to cash-in on the non-existent Episode V ½.
- Some have accused Avatar and the Airbending Fellowship of Vampire Slayers of being this. They are clearly insane.
- Some people claim that the Wizbits and the corresponding Elemenstor saga does not exist. They're just jealous mudkings who probably read Song of the Sorcelator.