That’s true, it’s rare, but it’s happened a few times here and there.
I immediately thought of the medical drama Trapper John, M.D., a spin off of the military/medical sitcom M*A*S*H (coincidentally, I’m wearing a M*A*S*H t-shirt at this very moment).
In the bizarre world of the M*A*S*H spinoffs, it gets weirder. Trapper John, MD is actually a spin-off of the movie, not the t.v. show, and there was some kind of lawsuit about it, but even more wierder, in the pilot there are references to the TV show anyway.