(And past crossovers aren't enough to stop references. The lead character of In Plain Sight cameoed on Law & Order: Criminal Intent before the show aired; but my understanding is that the writers of IPS later made reference to Law & Order as a TV show.)
Similarly, Mad About You
established that Paul used to live in the apartment now rented by Kramer, but Seinfeld
later showed George watching Mad About You
The same thing happened with John Munch appearing on The X-Files
, even though he had referred to it as a TV series on Homicide
I've never been a fan of those old '70s and '80s TV episodes where the heroes debunked a supernatural hoax but then there was a final gag that hinted "maybe it's real after all, woooo."
I've been rewatching Quantum Leap
lately, and this tendency has annoyed me in a similar way. Just off the top of my head, the series managed to imply the existence of alien abductions, ghosts, guardian angels, and vampires.