Lost in Space in 1968 seems my obvious choice. The series went from cool and cutting edge to audition’s for clown college. They doomed themselves: people stopped watching in droves.
Methinks Star Trek’s “The Trouble With Tribbles” is another example.
Any others?
“The Trouble With Tribbles” is Shakespearean in comparison with “Spock’s Brain”.
My personal choice would be “Happy Days”. I mean, it literally was the inspiration for “jumped the shark” as a name for the point when a series spun off the rails.
Also, “Mork & Mindy” (a “Happy Days” spinoff, no less)…“Mork lays an egg that hatches Jonathan Winters” has to be the definition of “off the rails”.
The first show that came to my mind was “The Drew Carey Show”. It started off as a normal-ish sitcom about a schlub and his friends, but they started getting into high-concept episodes and retooling the original idea.
Methinks you are very wrong
Australian TV soap Chances was canned during the broadcast run, allowing the writers to go crazy with neo-Nazi / hallucinagenic / high nude content / deus ex-machina intervention plot-lines to run out the series. Moving it out of prime time was the equivalent of pouring petrol onto a burning train-wreck.
Fondly remembered, never re-screened.
It happens often when 1) An actor in a leading role leaves, B) the focus shifts to an originally lesser character, or III) when there’s a shift in an overall theme of the show.
Examples:
B. Let’s see, there was Henry Winkler’s Fonzie becoming a co-lead in “Happy Days”, Jimmy “J.J.” Walker taking over “Good Times”, Steve Urkel taking over “Family Matters”.
III. Thematic shifts - “Life Goes On” was a drama about a cop married to a former flower girl in the post hippie era raising their family. The original theme was the family helping their son with Down Syndrome learn to be self sufficient, but then it drifted to the younger daughter’s boyfriend who had AIDS. “These Friends Of Mine” started out with Ellen Degeneres as the owner of a book store dealing with both her normal and her quirky friends. Then it was renamed “Ellen” and became about Ellen’s character discovering and accepting that she was a lesbian. And then there was the last season or two of “Roseanne”.
Some series can handle these types of change and some can’t. I guess it’s a flip of the coin.
Popular term for this is “Jump the Shark”
Ive noticed a lot of my favorite shows have NOT done this over the past few years, it seems writers and producers have learned from decades of great TV shows being run into the ground.
** Lost in Space** was never cool, and definitely not cutting edge. It started out dull and derivative, but eventually found its niche as a comedy.
Haven’t seen most of these examples, but I’m thinking here this isn’t pure “off the rails” stuff. A show I really like is NewsRadio, which started as a fairly normal workplace comedy, but it soon became clear that most of the characters were pretty insane, and rather than pretending they weren’t the writers embraced this. Sure, story-wise this means many of the later episodes are ridiculous, but it made the comedy work really, really well.
Also see “Flanderization” on TVTropes.
Less and less Carlin as the Bob Newhart Show went on.
I thought NewsRadio was really floundering after Phil Hartman was murdered. Lovitz’s character was just wrong.
It seemed to be absurdist comedy right from the start to me.
Ellen didn’t derail when it turned it’s focus to Ellen being a lesbian, but one of the actors expressed some regret that it turned into the ‘big gay show’ instead of a typical sitcom, and indicated that other cast members felt the same. I suppose that’s largely because the secondary roles were diminished at that point.
Some shows may have benefited from running off the rails, or at least switching tracks. I don’t know if Family Matters or Family Ties would have done as well as they did without becoming the Urkel and Alex P. Keaton shows.
It was a valiant effort to keep the show alive as a tribute to Phil. Lovitz did well, but the shock of losing Phil Hartman/Bill MacNeal made it impossible. Lovitz could have been a regular character in the show along with Phil, and he did play two parts before Phil’s death, but he didn’t work as a substitute. If Eva Gabor had died in the middle of Green Acre’s run the same thing would have happened to that show.
Rears up and squeals in horror.
Sleepy Hollow comes to mind. I thought the first season was well done. Fun and silly, but working in the scares. It’s been down hill since then. Now it’s just floundering around, completely disconnected from it’s original premise.
NewsRadio was probably running out of steam by the end of season 4 anyway, and John Lovitz and/or his character just weren’t as funny as Phil Hartman. “Rocket fuel malt liquor…” “The Real Deal with Bill McNeal”. 
Season 5 wasn’t a total waste of time, though.
Look, that episode is only popular under the “it’s so bad that’s it’s good” mantra.
No, that’s not true at all.
You may be the first person I’ve ever heard say that.
Mork laying an egg that hatched into Jonathan Winters was certainly off the rails, but I’m not sure it’s necessarily a bad thing. I mean, that whole show existed for the sole purpose of letting Robin William go off the rails, and it worked.
At one point, she was ‘coming out’ every week to somebody. When the week came that she came out to her plumber… I think the show met this thread’s criterion.