This is going to be a bit of a doozy, since it goes into a larger shared universe controversy that ultimately results in literally hundreds of shows becoming a fictional construct. To start, St. Elsewhere was a hospital drama with some black humor, basically a combination between M*A*S*H and ER.
In the course of its run, it crossed over with other shows, including Cheers and Homicide: Life on the Street. At the end of St. Elsewhere, there's a tight zoom on an autistic child playing with a snow globe that eventually coalesces into the exterior of the hospital that viewers had been seeing for years, implying that the entire show took place within the child's mind.
Fans took this to mean that not only was St. Elsewhere a product of the child's imagination, but so were all the other shows it crossed over with, including references in other live-action serial dramas like X-Files, Oz, and even Firefly. It's all painstakingly collected in the official site for the "Tommy Westphall Universe," named after the child seen playing with the snow globe. There might be other twists as controversial, but not many can profess to rendering 90% of all television a fiction within a fiction.