i know of no certain examples.
Possible Crossover Number One:
But I find a year number mentioned in "Space Seed" interesting, The teleplay is credited to Carey Wilber and Gene L.Coon, with Gene Roddenberry writing the second revised final draft.
When Kirk & Co. beam over to the Botany Bay:
SCOTT: Definitely Earth-type mechanism, sir. Twentieth century vessel. Old type atomic power. Bulky, solid. I think they used to call them transistor units. I'd love to tear this baby apart.
MARLA: Captain, it's a sleeper ship.
KIRK: Suspended animation.
MARLA: I've seen old photographs of this. Necessary because of the time involved in space travel until about the year 2018. It took years just to travel from one planet to another.
Marla McGivers does not specify whether the faster form of space travel at about the year 2018 (in the "Space Seed" calendar era) wa merely a much faster form of slower than light interplanetary travel or the first use of the warp drive.
Famed science fiction writer James Blish was hired to write adapations of *Star trek * episodes.
The first collection of those episodes, titled Star Trek, was published in January, 1967 according to the title page. So the staff should have known about the connection to James Blish while writing and editing various drafts of the teleplay for "Space Seed".
One of the most famous science fiction series written by James Blish was the "Okie" series or Cities in Flight. Stories dealing with the development of FTL interstellar travel in that series happen around the year 2018. James Blish wrote a chronology of the "Okie" stories, published in The Trimuph of Time, which says that the events leading to the first interstellar colonization voyage by humans happen during the years (AD) 2012 to 2021.
So it is no wonder that Blish's novel about the discovery of interstellar FTL travel in the "Okie" series, They Shall Have Stars, was also sometimes titled Year 2018!.
So it is usually assumed that the mention of "about the year 2018" in "space Seed" was an aknowedgement of James Blish's status as a famous science fiction writer and his connection to Star Trek.
James Blish mentioned a few persons, places, events, etc. from his own stories in his adapation of TOS episodes and his novel Spock Must Die!. That means that Blish's Star Trek writings happen in a fictional universe combining aspects of the Star Trek universe and aspects of Blish's "Okie" stories and some other Blish stories.
And naturally some Star Trek fans have imagined that the Star Trek televison series itself might possibly be in a fictional universe containing some aspects of Blish's own stories.
For example, the first chronology of Star Trek I ever saw, in the very early fanzine Star Trek: An Analysis of a Phenomenon in Science Fiction. 1968, mentions the war with the Vegan Tyranny from the "Okie" stories happening in its Star Trek timeline.
So I guess that a minority of Star Trek fans might want to consider "Space Seed" a sort of a crossover with James Blish's "Okie" stories. Even though we can't be certain that the year 2018 in "Space Seed" is the same as the year 2018 in the "Okie" stories.
Possible Crossover Two:
Famed Science fiction, horror, fantasy, etc. writer Robert Bloch wrote a few Star Trek episodes.
He penned three Star Trek: The Original Series episodes. All of these were based on short stories written by him previously. "What Are Little Girls Made Of?" came out from three of his stories, most notably "Queen of the Metal Men", "Catspaw" was loosely based on his 1957 story "Broomstick Ride", while "Wolf in the Fold" was adapted from the 1943 story "Yours Truly, Jack the Ripper", which was previously adapted to an episode of Thriller in 1961. Two of his episodes ("What Are Little Girls Made Of?" and "Catspaw") make mentions of the "Old Ones", a term frequently used by Lovecraft in his stories.
Robert Bloch also wrote a bunch of stories in the Cthulhu Mythos, a sort of a shared universe created by H.P. Lovecraft. Bloch and Lovecraft even wrote stories in which characters based on each other suffered terrible fates at the hands of eldrich monstrosities. The terms "Great Ones", "Old Ones", and "Great Old Ones" are mentioned in a number of Cthulhu Mythos stories by Lovecraft and others.
So it is possible for some people to believe that Bloch intended mentions of the "Old Ones" in "What Are lIttle Girls Made of?" and "Catspaw" to put those stories - and all of Star Trek by extension - in the fictional universe of the Cthulhu Mythos, and that the production staff of TOS went along with that idea.
Thus some fans might consider "What Are lIttle Girls Made of?" and "Catspaw" to be crossovers between the Cthulhu mythos and Star Trek.
Possible Crossover Number Three:
The Trek BBS, if I am allowed to mention a rival site, has at least one thread discussing possible Star Trek crossovers.
User ITDUDE created a thread called "Star trek is Part of MCU":
He linked to this site:
If Star Trek is part of the Marvel Cinematic Universe it would open the possibility of many crossovers with other fictional universe.
There is a TNG novel where the crew of the Enterprise-D meet X-Men from the fictional universe of Marvel Comics. Unless the Marvel Comics universe and Marvel Cinematic Universe are considered to be totally and 100 percent separate, with absolutely no overlap, the TNG expanded universe would be linked to the Marvel Cinematic Universe through its crossover with the Marvel Comics Universe, and Star Trek as a whole would be linked to the Marvel Comics universe though the possible connection with the Marvel Cinematic Universe.
And of course there have been crossovers between the Marvel Comics Universe and the DC Comics Universe, such as "Superman vs Spiderman", etc. Of course Green Lantern is part of the DC Comics Universe.
The animated Duck Dodgers tv series (2003-205) had an episode, "The Green Loontern", October 18, 2003, with a crossover with Green Lantern.
In Duck Dodgers Warner Brothers Loony Tunes characters Daffy Duck, Porky Pig, and Marvin the Martin portray Duck Dodgers, The Eager Young Space Cadet, and the Martian Commander. So some people could consider the whole series to be a crossover with every Loony Tunes production.
So some people could, by considering all those crossovers to be totally canon in all the affected fictional universes, believe that Star Trek and Loony Tunes happen in the same fictional universe. But every time I mention that possibility I also feel obligated to state the opinion that any Star Trek fans who think that would be good for Star Trek are "loony tunes".
To be continued later with even more crossover possibilities.
I have contributed a number of posts the thread:
In some of those posts I discuss various crossovers between expanded Universe Star Trek novels and other fictional universes. Those do not count for the purposes of this question.
But there are other crossovers and alleged crossovers and common universe elements and alleged common universe elements which do connect various Star Trek episodes and movies with other fictional universes.
Possible crossover number Four:
For exampled, there is the possible crossover between the sitcom Webster (1983-1989) and TNG. In the last episode "Webtrek", March 10, 1989, Webster seemingly found himself on the Enterprise-D and met Worf. Or maybe Webster only dreamed it, which would make Star trek fictional in the fictional universe of Webster (1983-1989), and thus doubly fictional, I guess.
As TV Tropes says, YMMV ("your mileage may vary") whether you consider "WebTrek" to be a proven crossover between Webster (1983-1989) and a part of the Star trek universe.
And it could get more complicated. TV Tropes has a trope called "Recursive Reality", which sounds complicated and can get very complicated.
"Recursive Reality" is complicated enough to have subtropes which elaborate on it in various ways.
And there is also the trope "Continuity Snarl", where the continuity of a work of fiction gets all snarled up.
The group of programs which TV Tropes calls the Nick Verse has examples listed under "Continuity Snarl":
The concrete canon nature that iCarly, Victorious, Drake & Josh and Zoey 101 exist in the same reality with each other, then having the cast of Victorious say that Drake & Josh was a tv show. This came the episode after they had Helen, a major character on Drake & Josh show up on Victorious and reference Crazy Steve.
As can be shown above, Big Time Rush existing as a fictional TV show and a real life band complicates things quite well.
And another example listed under "Recursive Reality"/"Recursive Canon"/"Mutually Fictional":
Probably averted (probably because of how complex these tropes are). Even though Drake & Josh exists in the same universe as the rest of the shows (as can be seen above) the Victorious episode Who Did It To Trina has the characters come out and say that Drake & Josh was a TV Show. Despite this, neither of these tropes probably (again, complex) apply.
So that is an example of a fictional universe where some tv programs are both fictional and real.
Thus proving that TNG is fictional in Webster (1983-1989) would not necessarily prove that TNG is not also real in Webster (1983-1989).
Too bad Kirk didn't have those examples to confuse the androids of Planet Mudd with.
Continued with more possible crossovers later.
Possible crossover number Five:
Here is a link to a page about Star Trek crossovers:
It lists many crossovers that many or most people wouldn't consider to be real crossovers, like Star Trek characters appearing in sketches in comedy variety shows, or aspects of Star Trek being fictional in various movies, tv shows, comic books, etc.
The earliest crossover it lists that some people might consider to be real crossovers is "Webtrek", March 10, 1989.
Then comes the TNG episode "Evolution", Sept. 23, 1989o which features a shuttlecraft Indiana Jones, indicating that Indiana Jones might be real and/or fictional in TNG.
An episode of Red Dwarf, Oct.7, 1993, has a Klingon Bird of Prey glimpsed.
The Millennium Falcon appears in Star Trek: First Contact and R2-D2 in Star Trek (2009) and Star Trek: Into Darkness.
Dr. Jackson Roykirk from "The Changeling" appears in the Team Knight Rider episode "Apocalypse Maybe", May 5, 1998.
A Constellation class starship is seen in the Battlestar Galactica (2004) episode "Daybreak", March 14, 2009.
Possible Crossover Number Six:
Another list of Television Crossovers is the Crossovers & Spin-off Master Page.
That site groups tv programs together in groups which share a common fictional universe according to the criteria used.
Their Group 10 includes:
Crossing Jordan (2001-2007)
Dalton's Code of Vengeance (1985-1986)
Knight Rider (1982-1986)
Knight Rider (2008)
Las Vegas (2003-2008)
The Office (US) (2005-2013)
Star Trek (1966-1969)
Star Trek: Deep Space Nine (1993-1999)
Star Trek: The Next Generation (1987--1994)
Star Trek: Voyager (1996-2001)
Team Knight Rider (1997-1998)
And if they update it in the future it will include:
Short Treks (2018-2020)
Lower Decks (2020-)
And future shows:
Strange New Worlds ()
Except that they don't list Star Trek: The Animated Series (1973-1974) and so probably won't list Short Treks, Lower Decks, or Prodigy.
And the only crossover between a Star Trek series and a non Star Trek series that they mention is between the Star Trek episode "The Changeling" and the Team Knight Rider episode "Apocalypse Maybe" with the character of Jackson Roykirk.
So all the other widely different shows in Group 10 are listed because of crossovers with Team Knight Rider.
Possible Crossover Number Seven:
Another site which lists crossovers is The Tommy Westphail Universe:
The creators of this site have different criteria for judging whether a crossover happens than the creators of the Crossovers & Spin-off Master Page.
The Crossovers & Spin-off Master Page lists 104 shows in Group 2 with St. Elsewhere (1982-1988).
But the site called The Tommy Westphail Universe lists a much larger group of shows that share the Tommy Westphail Universe with St. Elsewhere (1982-1988).
The latest update in The Tommy Westphail Universe site says:
With the definition of television expanding to include streaming content, and whole new platforms developing shows to entice people to subscribe, in 2015 there were 409 scripted series across broadcast, network and online services. It will be higher in 2016.
So now in 2021 there should be even more than 409 shows in the Tommy Westhphail Universe. So clearly the creators of that site used different criteria for judging whether shows have crossovers and/or share a fictional universe.
Here is a link to a grid of the Tommy Westphail Universe:
According to the grid St. Elsewhere is linked to programs including Cheers, which is linked to Fraser, which is linked to The John Larroquette Show, which is linked to Angel and to Star Trek: The Next Generation.
It shows Star Trek: The Next Generation as being linked to Journeyman (2007) and Webster and of course to the other Star Trek programs (except for the animated series and the post 2017 shows). It shows Star Trek (1966-1969) linked to The Red Dwarf and to Team Knight Rider, and Team Knight Rider linked to Knight Rider, Knight Rider (2008), and Code of Vengeance, but not to other programs that are in group 10 according to The Crossovers & Spin-off Master Page. They are linked in other sections of the grid.
It shows The Red Dwarf as connected to Doctor Who (1963-1989), Doctor Who (1996), and Doctor Who (2005-), to Space 1999 (1975-1977), and to Angel.
It shows Doctor Who (1963-1989), Doctor Who (1996), and Doctor Who (2005-) as being linked to Torchwood, Sarah Jane Adventures, *K-9 and Company *, Chelmsford 123, Hitchhiker's Guide to the Galaxy, and Power Rangers, and Power Rangers linked to Ninja Turtles: The Next Mutation.
The grid shows Angel linked to Firefly, which it shows liked to Battlestar Galactica (2003), which it shows linked to Caprica.
The grid shows St Elsewhere linked to programs including Homicide: Life on the Street which it shows linked to programs including The X-Files which it shows linked to programs including Buffy the Vampire Slayer which it shows linked to Andy Barker PI and Dexter, and of course Angel, which is linked to the Star Trek shows through The John Larroquette Show and Red Dwarf.
There are a few hundred other shows listed besides these.
So anyone interested in links between any Star Trek series and any other series listed in those sites will have to find the justifications for each link in the chain that leads from a Star Trek series to that other series.
If you don't agree with even a single one of the connections, you will have to give up linking those shows or else find another route between them.
Possible Crossover Number Eight:
Science fiction writer Philip Jose Farmer wrote in two books, Tarzan Alive: A Definitive Biography of Lord Greystroke (1972), and Doc Savage: His Apocalyptic Life (1973) that a number of famous fictional characters were descended from travelers in two stagecoachs who were exposed to mutating radiation from the famous Wold Newton meteorite in 1795, thus creating the Wold Newton Family.
Win Scott Eckert and others have elaborated the Wold Newton Family into the Wold Newton Universe, adding more famous fictional characters as ancestors and descendants of the stagecoach passengers or totally unrelated persons active within the same fictional universe.
And I certainly haven't been keeping track of how many different fictional characters, famous stories, and fictional universes have been claimed to be part of the Wold Newton Universe.
And there are sources claiming that Star Trek is part of the Wold Newton Universe.
One piece of evidence from a Star Trek movies and not from an expanded universe source is Spock's statement in Star Trek VI: The Undiscoverd Country:
SPOCK: An ancestor of mine maintained that if you eliminate the impossible whatever remains, however improbable, must be the truth.
The most reasonable interpretation is that Spock said that Sir Arthur Conan Doyle or some other human who said a similar phrase was his ancestor. But the most fun interpretation for many persons would be that the always truthful Spock said that Sherlock Holmes was his ancestor.
But several episodes of TNG claim that Sherlock Holmes was a fictional character, including "Lonely Among Us", "eEementry, Dear Data", and "Ship in a Bottle". If Sherlock Holmes is fictional in TNG he can not also be a real ancestor of Mr. Spock in Star Trek VI: The Undiscovered Country.
But As I quoted above, in the Nick Verse there are shows which are both fictional and real in other shows in the Nick Verse. So possibly Captain Kirk could have driven computers insane by explaining that Sherlock Holmes was somehow both imaginary and a real ancestor of Mr. Spock. If captain KIrk didn't go crazy himself trying to comprehend how that could be.
Possibly Sherlock Holmes was fictional in the universe of Star Trek as well as in ours, but a trans dimensional traveler brought Sherlock Holmes from a universe where Holmes was real to the universe of Star Trek where Holmes stayed long enough to have a child with a women there before the traveler returned Holmes to his fictional universe, and Spock found a record of it left by the traveler as a family heirloom while researching Amanda's ancestry.
After all, it is claimed that Star Trek is part of the Tommy Westphail Universe along with Doctor Who, and one episode of Doctor Who had him travel outside of reality to the land of fiction (which seemed real enough).
Therefore, some people might want to consider Star Trek part of the Wold Newton Universe.
Possible Crossover Number Nine:
I almost forgot Star Trek: The Animated Series which has been considered non canon, partially canon, and totally canon with the rest of the Star Trek universe by different people at different times.
The episode "The Slaver Weapon" was written by famed science fiction writer larry Niven, based on his short story "The Soft weapon", part of his kown space series. So Niven included aspects of the known space universe in "The Slaver Weapon", including the Slavers, stasis boxes, and the Kzinti.
so it is possible that "The Slaver Weapon" counts as a crossover between Known Space and Star Trek.
And I apologize for writing such a short discussion of possible Star Trek crossovers, but I have been working on it all day and I am quitting.