Is this correct: Easy Reader first appeared as a guest on Sesame Street before becoming a regular character on The Electric Company?
Somewhat related, Kermit, Ernie, and Bert went to The Muppet Show.
I think in all three cases, the actors (except for Stephen Hawking, who of course was not an actor) appeared as themselves on these shows.
One of the weirdest has to be Bernard Fox, who played Doctor Bombay on Bewitched up to 1972, returned to play him on the NBC Soap Opera Passions in 1999 (and sort-of played him on an episode of Pee Wee’s Playhouse in 1989.
Not that I know of. And I think I would know of it, if it were so.
Similarly, American Gods (sort of) has Lucy Ricardo as a character: One of the New Gods is Media, who most often manifests as Lucy, though it’s debatable whether she’s Ball or Ricardo.
No, it was in the episode Trials and Tribble-ations, during which the Defiant travels back in time to Kirk’s era during the events of The Trouble with Tribbles.
He also played a character on St. Elsewhere called Elliot, who was an inmate of St. Elygius’ psych ward. He, too, claimed that he was ruined by “some quack in Chicago.”
Yes, this is a whoosh. “Mr Anderson” was not only the name of the character on Beavis and Butthead, “Mr Anderson” is also a famous phrase of Agent Smith in The Matrix, referring to a different Mr Anderson, and the same actor, Hugo Weaving, played Elrond in Lord of the Rings. People sometimes inserted “misssster anderson” into Elronds quotes as a joke because they couldn’t help but seeing Hugo as Agent Smith.
That is a really obscure whoosh.
There was an obscure sitcom in the early '90s called Hi Honey, I’m Home which featured a 1950s TV family transported to the present. Among the many television characters that managed to stop by were: Grandpa Munster, Lisa Douglas, Alice the housekeeper, Alice Kramden, Dobie Gillis, Gomer Pyle, June Cleaver, Sally Rogers and a few others, all played by their original actors.
Another Newhart one: He played Professor Proton on The Big Bang Theory. Getting an Emmy. He’s so good that he appeared on it even after he died in-show.
He has appeared on TV in the prequel Young Sheldon.
But there’s overlap, maybe violating the OP.
The Bob Newhart reunion show – a continuation of the original Bob Newhart Show – had a cameo of Larry, Darryl, and Darryl at the end.
The story involved Bob talking about the dream he had night before about running a country inn.
Johnny Carson appeared as himself on an episode of Newhart
That I knew; I just couldn’t remember if the other three were in it as well.
Terry Farrell in a Starfleet microskirt, OMG! :o
Riker and Troi for Star Trek: The Next Generation both appeared in the much reviled finale of Star Trek: Enterprise long after Next Generation went off the air.
Robert Culp appeared on an episode of The Cosby Show when Bill dreams he’s a secret agent.
He also appears in an episode of Get Smart. It’s been too long since I’ve watched the show to remember whether he played himself or not.
The weirdest cameo ever has to be the episode of Batman where Werner Von Kepler appears as Colonel Klink.
Surely you mean Werner Klemperer? :dubious:
I don’t think either counts, since all four shows were on concurrently.
Is Johnny Carson playing himself the same as an actor playing a character? 
Johnny Carson played the conductor on a train going through Central Europe. Not himself, unless he had a past I was unaware of.
I don’t think this falls into what the OP had in mind.
It’s possibly stretching things a bit, but George Lazenby appears as a British secret agent driving an Aston Martin with plates reading “JB” in the TV movie The Return of the Man from U.N.C.L.E.
They don’t explicitly say he’s James Bond, but it’s about as close as they could get. And, besides, it’s not a TV character, but a movie character.
I’m assuming it was also a part he played only once?