The Star Trek: Deep Space Nine cast appeared as the staff of a Fifties pulp sf magazine in one episode; the covers of several of the magazines were drawn from earlier ST art: Incredible Tales of Scientific Wonder | Memory Alpha | Fandom
Tom Paris enjoyed Holodeck adventures as Captain Proton on Star Trek: Voyager, so he was a sf character pretending to be another (much earlier) sf character: Captain Proton | Memory Alpha | Fandom
It is more complicated than that. Green Hornet and Kato first appeared in a bat-climb in episode #41 “The Spell of Tut”. Batman and Robin recognize them as fellow crime fighters although Robin is puzzled by their weird outfits. Later they starred in a two part episode “A piece of the action”/“Batman’s satisfaction” where like most people Batman and Robin think the Green Hornet and Kato are criminals (Britt Reid is an old friendly rival of millionaire Bruce Wayne).
An approach also used in Stargate SG-1 with Wormhole X-Treme!
When the Good Girls did their money laundering-shopping, they did so at a Cloud 9 store, the location of Superstore.
Jay Sherman of The Critic appeared on The Simpsons.
It stunk.
I’ve heard this one before, along with the resolution.
Jerry Seinfeld still plays a relatively famous stand-up comedian in his show. All that is required is that he’s done a TV special, and it all makes sense. It’s still Jerry from the show, not Jerry from real life.
I don’t know about the OP. Do they play the actual same characters from the show in their play? Could the play be based on a true story in that universe?
Although, as I mentioned, Mad About You is a show that George and Susan watch in the Seinfeld universe. Basically, they just don’t worry as much about those kind of continuity issues.
You can say that again.
There was an episode of St Elsewhere in which Dr Craig has just banged his wife and decides to turn on the TV to “see who’s on Carson; probably some stupid comic.”
We can’t see the tube, but the sound comes up and it’s clear he’s watching Howie Mandel (Dr Fiscus) do his trademark routine.
“What’s that he’s wearing? A giant hand?”
Community/Cougar Town/Scrubs.
Abed from Community talks about being a fan of Cougar Town on several occasions, so Cougar Town exists as a TV show in the Community universe. IIRC Abed wrote to the Cougar Town producers and wound up being able to visit the set. He says they gave him a background part. And indeed, he can be seen in the background of this Cougar Town episode.
And crossing over the other direction, the characters Travis and Laurie from Cougar Town have a very brief appearance in a Community episode.
Also many actors from Scrubs have roles on Cougar Town. But during a couple episodes there was an almost complete crossover although the characters weren’t officially the same other than Ted making references to the people at his old job: Cougartown finally collides with Scrubs - YouTube
Required link in all threads of this type - Fan Theory Proves Almost All TV Shows Exist Within Same Universe | HuffPost Entertainment
Petticoat Junction, Green Acres, and the Beverley Hillbillies are all on the chart.
Sometime around the middle of their runs, I noticed that St Elsewhere and Hill Street Blues often borrowed plot elements from each other. This was logical, since they were both MTM productions. The one I remember best is the male stripper hired for someone’s birthday. On HSB, it was for Lucy Bates; a couple of weeks later, it turned up on SE: “I got the idea from some cop show.”
St Elsewhere used other MTM elements as well. A mental patient mistook the Navy officer played by Betty White for Sue Ann Nivens, and (if I’m not mistaken) one of Bob Newhart’s psychiatric patients showed up complaining about his former analyst, “a real quack in Chicago.”
I like to think the characters in that episode are the real ones, and that DS9 is all fiction.
But then, I think Buffy is actually in a mental asylum, so…
Archer had a crossover epidode with Bob’s Burgers, which was weird since H Jon Benjamin voices both Archer and Bob. The artwork for Bob’s Burgers was redone Archer Style, so you had Archer as Bob, looking different but sounding exactly the same.
Jack Bauer watched Lou Grant? :dubious: 
The character “Warren Coolidge” originated on “The White Shadow” but later was a regular on “St. Elsewhere”. In one episode of the latter show, the actor who portrayed “Salami” guested in a different role and “Coolidge”, upon seeing him called out “Hey, Salami”.
On an episode of The Simpsons, the Futurama cast travels back in time, and then the Simpsons cast travels forward.
However, in the Futurama episode “A Big Piece of Garbage,” the garbage ball includes Bart Simpson dolls, implying that he is fictional.
Also, orderly Coolidge mistook a character Tim Van Patten was playing for “Salame,” someone Coolidge played basketball with on The White Shadow.
But “St. Eleswhere doesn’t count as it was all in Tommy’s mind the whole time” - otherwise, I would point out that there was one episode where Dr. Morrison went to the bar that was the inspiration for Cheers, and there is a banner outside of the bar saying as much, but in a later episode, the Cheers bar, and its characters, are real.
On Mad About You they established that her mother had a bit part as one of the pretty girls on the Alan Brady show, which is the not-Carl Reiner sketch comedy show that Dick Van Dyke worked on in his sitcom.
Also on MAY they met John Astin playing Gomez Addams although they didn’t name the character. Given that Lurch appeared in one episode of Batman (in a window gag: “Return to your harpsichord good citizen”) that tells us that Friends is set in the same universe as Batman.
That wouldn’t surprise me. She killed all the vampires in Beverly Hills, and watched her mentor die. That kinda trauma might send anyone to the loony bin, where she can delude herself into thinking that she’s continuing those activities in a different town for ten years without hurting anyone important.
I’m not sure if these count, but there was an episode of The Cosby Show where he dreams that he’s a spy and Robert Culp shows up.
There’s an episode of The Drew Carey Show where Drew Carey’s mother (June Lockhart) is watching Lassie on TV.