In the final episode of the 80s TV show St. Elsewhere, it was revealed that the entire series had taken place in the imagination of an autistic child. So therefore, the other 12 series that crossed over into the St. Elsewhere universe were also born in Tommy Westphall’s mind. And so were their crossovers and spinoffs. Here's a huge graphic that shows how all those series (282 of them) were related. Link -via Kottke