What's the most surprised you've ever been at a (non SIMPSONS) TV guest star?

Trebeck wasn’t their first choice. They wrote the scene for Johnny Cash so that there would be the obvious joke of Mulder trying to convince people that Johnny Cash was a Man In Black.

Sean Penn did a hysterical cameo on Ellen soon after her coming out episode. In the episode Ellen was working as personal assistant to Emma Thompson, who played herself, kinda- she was Emma Thompson the actress, closeted lesbian and lush who it turned out had one other thing to “come out” about. She’s not English, but from Dayton, OH and has been faking an English accent for her entire career.
Through Ellen’s encouragement Thompson decides to come out publicly at an awards banquet. Unfortunately Sean Penn (as Sean Penn) accepts an award via video at the same banquet, and beats her to the punch by coming out, and since he’s the bigger star it steals her thunder. Hilarious cameo that I’d forgotten til I saw his name in here.

I didn’t know until recently that Davis made a return visit on Archie Bunker’s Place. The entire episode is on YouTube and is mostly missable, but the last minute is good and showed how much Archie had grown as a person.

I’m gonna show my youth here…
Why was this so surprising?

Maybe the fact that it had been twelve years since anyone had seen the Hartleys, and as far as current watchers were concerned, this was Dick Louden waking up, not Dr. Bob Hartley.

To flesh that out some more, the second sitcom starring Bob was shown to have been a dream for Bob’s character in his first show. This is shown by his waking up next to his first tv wife, meaning he’s really the character from the first show.

No.

THE greatest surprise guest-shot ever.

Which itself was a take off on the shower scene from Dallas (ie the entire second Newhart show was all a dream)

I don’t think that is true unless you heard someone from the show verify it. The “It was all just a dream” bit is much older than Dallas.

Most surprised? That has to go to an episode of Law & Order: Criminal Intent in which the main guest star was…

…Stephen Colbert, playing a totally straight dramatic role and doing it well.

He’d already done Strangers With Candy and at the time was currently on The Daily Show, so I was pretty well surprised when I saw him turn up on L&O.

For me, it was Brad Pitt on Friends, tackling the role of a nerdy, unlikeable character.

Yeah, he was married to Jennifer (why, oh why would he leave that beautiful woman for that Jolie person??), but still…

Keith Carradine on Criminal Minds, playing a sociopathic serial killer who did really weird things with his victims bodies. He played it just absolutely right.

Bob Newhart had two very successful series years apart. In his first series as a psychiatrist in an urban setting his wife was played by Pleshette.

His second where he played an inn keeper in Vermont with a different actress for a wife lasted a few years and on its final show they ended it with a closing scene of Bob and Pleshette waking up in bed in their urban apartment and Bob telling her that he had the weirdest dream.

Just about everybody who watched his Inn Keeper show was familiar with his previous series and so the surprise was a very powerful gag.

Moved and seconded.

Wait, was this inspired by Hodgman on BSG?

(Although that was not so much ‘surprising’ as ‘unfortunately takes you out of the moment’)

Not the greatest, as that’s already been mentioned, but I loved Sean Bean’s cameo on The Vicar of Dibley in Dawn French’s dream sequence.

It has a long and disreputable history in literature and has been used as a “reset” at the end of single episodes, but I’m pretty sure that Dallas was the first TV series to wish an entire season into the cornfield like that. It caused quite a stir at the time and has often been the subject of parody.

Don’t think I was claiming that Dallas was the original concept, just that it was a tip of the hat as it were. As your cite notes, it’s a standard ploy, but the Dallas rendition was seen as a particularly egregious example of it.

Well if you want to get into the creepy guest shots:

on L & O SVU we had Martin Short playing a very creepy sexual predatory killer of virgins, Fred Savage doing a marvelous job as a rapist who seemed like ‘just a great guy’

I associate it most with the Wizard of Oz (The 1939 classic movie) but to each there own.