The classic crossover started itself off as a good way to make the best of what you've got — so if you have two shows on your roster, it's a no-brainer to have the shows and characters cross over every once in a while, especially if one of the shows is less popular than its sibling.
The popular way of doing this is the "true" crossover, in which a storyline will begin in one series and cross over into the next one, encouraging viewers to tune into a show that may be thematically similar but which they do not usually watch.
For example Buffy the Vampire Slayer had a handful of episodes where the characters would head off at the end, only to turn up in Angel straight after. This also worked the other way, with a magic amulet in Angel turning out to be vitally important for the last-ever episode of Buffy. (This also highlights one of the few dangers of crossovers; if you or a fan watched only the Buffy show, but not Angel, this particular amulet appeared to come totally out of nowhere to save the day....) Sometimes this is done to provide the lead-in for a spin-off show, as happened with both of the CSI: Crime Scene Investigation spin-offs.
Alternatively, a single popular character can cross over from one show into the other, for a brief guest appearance; this has the effect of attracting that character's fans from the other show without requiring the writing teams to sync up or creating DVD- and arc-unfriendly episodes. This is very common in comic books, in which most characters are part of a larger universe, such as the Marvel Universe or The DCU. It happens less often in TV and movie properties based on comic books, since they are often made by different production companies.
As more ways of developing connections between shows formed, we ended up with lots of different subtropes of crossover. All of these can be seen on the Crossover Index and any examples for those will be found on their own pages.
Not to be confused with certain SUVs.
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Anime and Manga
Go Nagai has frequently crossed their different works (mainly his Humongous Mecha series) over since the seventies. Some of the main instances are:
Mazinger Z, Great Mazinger, UFO Robo Grendizer, Getter Robo, Getter Robo G, Devilman and Cutey Honey in the Dynamic Heroes e-manga (also known as Nagai Go Manga Gaiden - Dynamic Heroes or Go Nagai manga heroes crossover collection - Dynamic Heroes), an e-manga released in 2004 and later compiled in tankoubon.
Getter Robo and Devilman in the "Devilman vs Getter Robo" manga. At the end of it, Akira and Miki lampshade the trope, joking about whom they will visit the next time.
For the 9th anniversary of the MMORPG MapleStoryTower Of God author was asked to make a vignette in which each property's respective heroes faced off against each other.
The mangaka group CLAMP have crossed their works over to the point that now they are all in one big, weird world. Some of these are significant to the plot, many seem to be just for fun. A frequent crossover element is the Clamp Campus, a gigantic private school that shows up in CLAMP Campus Detectives, X/1999, Dukylon: CLAMP Defenders, and Man of Many Faces. Characters from Chobits make an appearance in Kobato. The protagonist of Tokyo Babylon, Sumeragi Subaru, shows up in X/1999. Even their "joke" series Miyuki-chan in Wonderland contains crossovers. In one story, Miyuki ends up as the Damsel In Distress in X/1999. In another, she ends up playing (strip) Mahjong with other characters from CLAMP works. And there are the tribute music videos CLAMP in Wonderland and CLAMP in Wonderland 2 which have segues between all of their series to date.
Then there is Tsubasa: RESERVoir CHRoNiCLE. The entire basic premise is being an Alternate UniverseMassive Multiplayer Crossover with characters from most of CLAMP's works showing up, often in different contexts from their original portrayal, such as characters from opposing sides of X/1999 hanging around the same cafe. Two of the main characters are alternate versions of the main characters from Cardcaptor Sakura. AND it is the sister series to Xxx Ho Lic. The plots of the two series are so intertwined that they are essentially two vantage points on the same story.
There are also separate anime and manga crossover specials of Toriko and One Piece.
And now there is going to be an OVA crossover with all three.
In the manga Digimon V-Tamer 01 are a total of 3 non canon crossovers, each being about a hero of the current digimon anime series being space warped and ending up battling an extremely powerful digimon together with Taichi and Zero.
In honor of Shounen Sunday's 50th anniversary, the publisher released a DVD including three OVAs for their three most popular animes: InuYasha, Urusei Yatsura, and Ranma ˝. The DVD also included this animated short, where the main characters of the three series met up. Hilarity Ensues.
Voltron: The Fleet of Doom, a TV special made for American audience which had Lion Voltron and Vehicle Voltron working together to battle the allied forces of King Zarkon and the Drule Empire.
And Doctor Slump's 1997 remake anime has Goku visits Penguin Village. The different is that Arale is now a main character while Goku is a guest star, and Art Evolution.
In one episode of the 1980s Astroboy, the titular robot is taken back in time, along with his sister Uran, the one and only Black Jack and the doctor's assistant/companion Pinoko, in order to save the main character from ''Princess Knight. Good ol' Tezuka mix-and-match.
Leiji Matsumoto's works. Like with CLAMP everything is in the same universe (Yamato and Harlock and Maetel and Emeraldas and the Queen Millennium...) but details do not match (Tenshiro died in three different ways already).
Pretty Cure All Stars crosses over all the existing Pretty Cure series in one big movie. This has gone from one movie teaming four teams to five movies featuring eight teams.
∀ Gundam acts as one for all of the Gundam shows that came before it. Universal Century mobile suits are excavated and used by the protagonists while newer mobile suits use technology from Universal Century. And near the end of the series, the Black History is revealed to be every single Gundam series, both before and after Turn A.
All of the shows by AIC makers of Tenchi Muyo! crossover some what with each other Dual! ended up with an cameo and recycling the "Light hawk wings" for its big mech. later in Tenchi Muyo! GXP and mech looking like the one from DUEL! showed up in an episode.
The authors of Fairy Tail and Flunk Punk Rumble did a one-shot called Fairy Megane that brought together the main characters from the two series. (The mangaka of the latter used to work as an assistant to the mangaka of the former.)
One Piece and Toriko have crossed over three times so far, the first special (which was animated) took place pre-Timeskip in One Piece, while the other two take place post. Both authors are real good friends so theres a chance that the crosses are canon.
Archie Comics is not afraid of this. The only real thing that defines the universes (excluding Sonic the Hedgehog and other licensed series) is different towns in America. The Archies performing alongside Josie and the Pussycats? Sure!
Sabrina however did crossover with Sonic the Hedgehog, in a two part-special that expanded both series.
In fairness, the early 90's Frank Castle was essentially a well-equipped assassin, not an amoral wholesale slaughtering machine leaving dozens of bloody corpses everywhere. And he not only engages in less violence here than his usual missions of the time, he even lets his target live. Still, the fact that everyone involved managed to pull it off at all, much less make a pretty good story, is downright amazing.
José Carioca's universe is set in his home country of Brazil, but he's shown to be friends with Donald Duck in Saludos Amigos and The Three Caballeros (as well as the ride in the Mexican pavilion at Epcot). Thus, there are occasional crossovers between the Carioca universe and the McDuck Universe. Indeed, there has been a prolonged arc of stories where José visits Duckburg, and interacts with the various characters there.
Various Image Comics titles (Invincible, Savage Dragon, Witchblade, etc.) occasionally cross over, but the editorial policy is that what happens in one title doesn't have to affect events in another. Each hero lives in his or her own unique universe that may or may not contain versions of the other heroes. For instance, Invincible's universe has a Savage Dragon and a Witchblade who are more or less the same as their counterparts in their own books and have roughly the same adventures, but what happens in Invincible's book stays there.
The very first superhero crossover was a battle between the Human Torch and the Sub-Mariner in Marvel Mystery Comics #8-9 (1940). This is the beginning of the Marvel Universe.
Actually, the Shield and the Wizard at MLJ met earlier. The Wizard had learned of an impending attack on Pearl Harbor and contacted the Shield. They spent the next few issues fighting the same enemies (the Soviet/Nazi proxies called Moskovians), occasionally crossing paths for a panel or three.
Superman and Batman have been appearing in comics together since the Silver Age, and Wonder Woman joining in isn't uncommon, either. Generally they team up, though having them fight each other is always a good way to drawn in readers.
Superman: "We're surrounded, you know. I can hear them all."
Batman: "I think we can take them. Do you think we can take them?"
If that counts, we may as well mention All Star Comics #3, the comic which createdThe DCU as a shared universe.
Usagi Yojimbo and Teenage Mutant Ninja Turtles had several single-issue gag crossovers in their respective early runs, which culminated, several years later, in a volume of Usagi which centered around a village shaman summoning the Turtles to fight invading ninjas, and the Turtles' subsequent confusion at being landed in a medieval Japan inhabited by talking animals.
At one point, they also did a string of Slice of Life segments between commercial breaks where all of the characters were shown coexisting in some sort of vaguely defined urban setting.
G.I. Joe and Transformers. While the TV show is limited to the "Old Snake" thing and the hologram of Marissa's dad (see Western Animation below), the comics have crossed over many, many times, even to the point of very important and far-reaching events in one starting in crossovers such as Cobra being the source of Megatron's tank body, and Bumblebee getting blown up real good and later repaired as his new Goldbug form - though if you're from the UK, you got a different Goldbug origin.
The various Marvel-DC crossovers that have been published over the years, which imply the existence of a third timeline separate from the Marvel and DC continuities in which both exist alongside each other.
There was also the Amalgam Universe, which consists of heroes who merge traits of a hero from each universe. Examples include IronLantern (take a guess) and Darkclaw, who was Batman and Wolverine (!!!). However, the events of JLA-Avengers are taken as canon for both publishers in their mainstream universes.
In the 90's, DC and Image published a crossover between Batman and Spawn. It's best remembered for completely rejecting the usual "meet, fight, become friends" routine found in most crossover comics. The two heroes hate each other for almost the entire story. At the end, Spawn wonders aloud if they could try to be friends, and Batman responds by burying a batarang in his face.
That is the main premise of the IDW comic Infestation: Outbreak, where a dimensional gate (which looks suspiciously like a Stargate) unleashes hordes of zombies from another world. Unlike other zombies from this world, these are guided by a single will, the Undermind. They are also somehow capable of infecting technology and other undead creatures. When an elite vampire squad is sent to deal with the situation, one of them, Britt, gets bitten and becomes a vampire/zombie hybrid. Britt uses artillica to open four more dimensional portals, through which the zombies and infected machines pour, along with parts of Britt herself. While the other vampires manage to shut down the portals, the threat has already spread to other worlds. These worlds are those of G.I. Joe, Star Trek, Transformers, and Ghostbusters. All these worlds now have to fight an infestation from a virulence they have not seen, infecting both man and machine.
Infestation 2 had the Elder Gods invade the IDW Teenage Mutant Ninja Turtles, G.I. Joe, Dungeons and Dragons, Bat Boy, 30 Days of Night, and the Transformers: Hearts of Steel universes by creating a rift through universes through H.P. Lovecraft's writings.
Bongo Comics produced a mini-series called The SimpsonsFuturama Crossover Crisis. The first half is the Planet Express crew being sent into an issue of The Simpsons by the Brain Spawn. The second half is the residents of Springfield being pulled into New New York by one of Professor Farnsworth's inventions. And then it gets weird...
In the German comic Werner: Dex & Dogfort appear in Volle Latte!, drawn by Jörg Reymann himself. In retour, Werner appears in the third Dex & Dogfort book, Schlachthofgiganten.
In the story about Wilhelm Busch's 150th birthday at the end of Alles klar?, Werner and Wilhelm Bush let loose his characters from Max Und Moritz because they find the celebration boring.
A favourite for many seems to be Doctor Who, due to the remarkable ease with which the Doctor can be inserted in any story just by having the TARDIS land there. One of the most famous, The Man With No Name, is a Firefly / Doctor Who crossover, which some say should be officially added to Canon.
Yet it VERY strangely lists one of the unofficial rules of the internet dealing with crossovers (specifically, rule ⑨
Star Wars and Star Trek. The rivalry between their fanbases and The Ultimate Showdown of Ultimate Destiny makes it a bygone conclusion feature some, and for those to require at least one major battle between the United Federation of Planets and the Galactic Empire, but there are certain exceptions.
Another good one is Unity by notable contributor SF Debris. What is a wormhole was connected from the Delta Quadrant circa Voyager's fifth season to the Star Wars Universe circa early Return of The Jedi, (disrupting the Rebel Alliance's attempt to attack the second Death Star.) Involved every character from TNG forward moving well into the future and exploring all the potential interactions in detail.
To complete the trifecta of renowned Star Wars vs. Star Trek fanfics, there's StarCrossed.
Shinji and Warhammer 40K is sort of an odd example. It is not a story where the universes meet, being more of a For Want of a Nail story about Shinji getting a large amount of Warhammer 40000 pieces and rulebooks as a child. His different (happier) childhood makes for a more dynamic protagonist and leads him to influence the rest of the cast into making things that fit the Warhammer 40000 ethos more than the original Evangelion series did. It's not the straightest example of a universe to universe Cross Over.
A more straightfoward example of a mostly in-universe crossover for Evangelion/Warhammer would be Thousand Shinji.
Brother Bubbles. Space Marine Terminator finds himself in Rapture. Then meets a Little Sister.
In Anime fanfiction (and sometimes in other media) a quite common way to establish a plot, especially in Peggy Sue fics, is a wish granted by Belldandy or her sisters. Whether this crossover is minor or major varies between fics.
The Ranma ˝ fandom loves to create crossovers. There exists several archives devoted to them. Particularly common are crossovers with Sailor Moon which has a sub genre called the Fuku Fic that has more fan fics written for it than most anime has fan fics total.
This fanfiction crosses over Sonic the Hedgehog with the cast of My Little Pony: Friendship Is Magic, in a reimagining of the plot of Sonic Generations. It goes far beyond what the original game gives, showing the author did its research. For instance, Ponyville serves as the central hub, similar to the White Void in the game, the Death Egg Robot battle in the original is replaced with Nightmare Moon and in this fanfiction, Tails fights Metal Sonic instead of Sonic, possibly a reference to Sonic the Hedgehog 4. The author has even adapted the updated canon of Friendship is Magic into the fanfic. However, how the author intents to implement alicorn Twilight is currently unknown.
An author on F Fnet has created this  a Star Wars and Halo crossover where the characters of Republic Commando (and some friends and enemies) meet Master Chief and company in the Halo universe. There is a sequel that takes place mostly in the Star Wars universe during the events of the Original Trilogy.
Bayonsiders has War from Darksiders and Bayonetta from Bayonetta meet up, have sex, beat up angels, beat up demons, and have sex while beating up angels and demons at the same time. All while trying to stay in continuity of both games.
Decent Pokemon/Digimon crossovers are not that common in fanfiction, but of late some interesting new submissions have popped up. Sturgeon's Law may be applicable due to grammar issues, but those with decent grammar tend to be alright.
Even though the first story of A Sticky Situation remains solidly in the Digimon season three universe, the second story tosses several of the characters into Star Wars: The Clone Wars due to a dimension-traveling digimon called Parallelmon (a Canon digimon, not an OC of the author). It is very well written and very entertaining.
When With Strings Attached was first conceived, it was supposed to be a Beatles/Dungeons & Dragons crossover. Luckily, the author quickly matured out of that notion, and the story turned into something far more interesting and complex, though its RPG roots are quite evident still.
There are a lot of fanarts involving Kaa the Snake hypnotizing various victims. Google search "Kaa" and you'll see what the deal is.
The longtime crossover champion for TV shows is "The Witness" by Michele Martin, which managed to cross Forever Knight, Highlander, The X-Files, Quantum Leap, and the Starman TV series. Believe it or not, the fic manages to keep all those balls in the air successfully, and even bring in cameos from a few other series, all while following a coherent plot and keeping the characters in-character!
Too many Godzilla movie crossovers to list as many of his adversaries (including King Kong and Mothra) had starred in their own films previously. He even had a smackdown with his own the American Godzilla (from the 1998 film) in Godzilla Final Wars.
And now, Disney has done it again with Wreck It Ralph. Although all the characters from existing videogames usually appear for a short period of time, it still counts.
The View Askewniverse movies are all connected, mostly by Jay And Silent Bob appearing in every movie. Aside from that, there are numerous events, elements, and secondary characters that intertwine throughout all six films. It also helps that the movies take place in the same state.
Long before the Marvel comics put two Robert E. Howard barbarian heroes together, Howard himself did it: in the Bran Mak Morn story Kings of the Night, Kull of Atlantis makes a special guest appearance. He and Bran team up against the Roman Legions invading Britain.
The 1995 novel H: The Story of Heathcliff's Journey Back to Wuthering Heights crosses over the two Bronte sister classics Jane Eyre and Wuthering Heights.
Just about every Sandra Dallas book includes mentions of characters from previous books. Many times, the connections are so minuscule, it takes a discerning reader to spot them.
Two minor Percy Jackson and the Olympian Characters, Drew and Lacy appeared in the Serpent's Shadow, after two books worth of hinting at the Greeks and Romans.
The book Garfield's Christmas Tales contains a story in which Garfield, during a Christmas trip to the Arbuckle family farm, pays a visit to US Acres (which happens to be right next door to the Arbuckle farm) and helps Orson, Wade, and Roy win a snowball fight against Orson's brothers.
Live Action TV
The "Tommy Westphall Universe" (or, alternatively, "The John Munch Principle") ties dozens of television series from the 1960's to the present through the characters of Tommy Westphall from St. Elsewhere (who is revealed to have imagined the entire series in the final scene of the final episode) and John Munch, the Baltimore detective who has made crossover appearances in many television series (including the Law & Order canon, The X-Files and Homicide: Life on the Street). Thanks to Munch's crossovers and cameos, as well as shared elements and names that occur between different franchises and series, a theorem was developed showcasing the connections. More than 90% of all television shows are connected via crossover, including the entire All in the Family franchise, The Wire, the entire Law & Order franchise, every major medical drama made in the 90's and 00's, and many more. Here's a handy chart for those keeping track◊.
And this isn't even the full extent of it, since the list doesn't include ANIMATED shows that have crossed over with these series (notably a brief crossover between the Simpsons and the X-Files) increasing the cross overs to just about 99% of all series ever put on television.
The episodes of Law & Order: Special Victims Unit and Law & Order: Trial by Jury titled "Night" and "Day," respectively, were crossovers of each other on the same night. The major characters from each series appeared on both episodes, which dealt with a rapist. SVU focused on the investigation and arrest of the rapist, while Trial by Jury focused on the trial. (There was some discontinuity here. At the end of SVU, the case was declared a slamdunk. At the beginning of TBJ, the case became a sure loser.)
Law And Order: SVU did this years ago with a 2-part event, "Entitled," the second half of which took place on the original Law & Order. However, this plays havoc with syndication, because the shows are rerun on different networks in America; the SVU episode leaves the viewer hanging without the second half.
Law & Order also crossed over with Homicide in what was originally a two-parter. While working together on a single case, the two squads had personality clashes, and normally-infallible characters faltered in unfamiliar settings. In a good move, when this episode is rerun TNT will show both the L&O and Homicide episodes back-to-back, even though they don't usually show Homicide at all.
Community has a great one, involving Cougar Town. Abed mentions that he was allowed on the set of Cougar Town as an extra, and that episode actually exists!
The whole point of moving Frasier to Seattle was to prevent network pressure for guest stars from Cheers. They couldn't hold out forever, though.
The character of Captain Jack Harkness first appeared in Doctor Who near the end of the 2005 series. At its finale, he was left behind by the Doctor, and was next seen in 2006 starring in his own spinoff, Torchwood. Torchwood ended its first series with Jack running away to rejoin the Doctor, which occurred near the end of Doctor Who's 2007 series in the three-part finale, after which Jack headed back to his team for Torchwood's second series.
Jack returns for the series 4 finale, along with Ianto and Gwen. The episode crosses over even further with the return of Sarah Jane — accompanied by her adopted son from her own spinoff series.
Likewise, the Doctor has had a guest appearance in The Sarah Jane Adventures, as has the Brigadier (making him the first Classic Who character to appear in a Nu-Who spinoff, but not Nu-Who itself). Another SJA crossover aired, with Eleven and Jo Jones.
Martial Law has the appearance of Chuck Norris' character for a couple of episodes. In fact, for that case, you have to watch the continuation of the case in Walker, Texas Ranger if you want to know how it is resolved.
Stargate SG-1 and Stargate Atlantis have had several instances of crossovers. Among the most notable are the characters Rodney McKay and Elizabeth Weir, who were introduced as secondary characters in SG-1 and became two of the first regular characters on Atlantis (later joined by other SG-1transplants.) Often times, things that happen in one series have an impact on the other, such as the recovery of a ZPM by SG-1 helping to defend Atlantis during its siege.
There was a dedicated episode where SG-1 goes to Atlantis to find out the secrets of the Deus ex Machina that'll save the galaxy, in the process both teams save the other one from disaster by the series' two Big Bads. Additionally, several episodes of Atlantis have included scenes set in the SGC.
Colbert and Jimmy Fallon each had the other crossover in the promotion of a fundraiser competition. It culminated in a performance of Rebecca Black's "Friday" that contained nearly the legal limit of awesome.
Casualty and its spin-off Holby City have had four crossovers titled Casualty@Holby City (although they were given a special title, they ran as two-parters in the usual slots): two Christmas Episodes, one Halloween Episode, and a Very Special Organ Donation Awareness Week episode. We have yet to see Casualty@Holby City With Holby Blue, but doubtless it's coming...
And it has: although it wasn't called Holby Blue@Holby City, the first episode of Season Two of Holby Blue was a direct continuation of that week's Holby City, both of which featured the medics and the cops.
The various Star Trek series saw many of these, beginning with The Next Generation, although events in one series rarely affected the others. The crossovers became more frequent in later years.
CSI: Crime Scene Investigation has crossed over with both other shows within the CSI franchise, CSI NY,CSI: Miami and also with Without A Trace. Miami, meanwhile, has crossed over twice with CSI: NY, and New York once also did so with Cold Case, and the three core CSI programs have crossed over with a three-part story featuring Ray Langston. All five are produced by Jerry Bruckheimer and air on CBS, so bringing them together is less complicated than most.
All My Children, One Life To Live, and the defunct Loving were all created by Agnes Nixon and share a universe. Numerous crossovers have occurred over the years. In 2004, AMC and OLTL launched an ambitious crossover storyline in which a baby from All My Children was kidnapped to One Life to Live. The story lasted several months. Traditionally, ABC considered all of their soaps to share a universe, which allowed for the 1999 story (essentially a ratings stunt) in which Linda Dano's One Life to Live character traveled to All My Children, Port Charles, and General Hospital searching for her long-lost daughter, who turned out to be a fellow soap-hopping character, Skye Chandler.
Especially between Port Charles and General Hospital, which were set in the same town and hospital. Characters from one show frequently appeared on the other, and major eye-candy events like the annual Nurses Ball would start on Port Charles and finish on General Hospital. Only when Port Charles took on a more supernatural tone did the crossovers stop.
Similarly, on CBS, The Young And The Restless and The Bold And The Beautiful were created by William Bell, leading to many crossovers and shared storylines. Though there are occasional glitches when actors who played one character on on show turn up as a completely different character on the other.
In a desperate attempt to prop up the ratings of its woebegone late '70s sitcom Hello Larry, NBC had the cast make numerous crossover visits to the popular Diff'rent Strokes.
Happy Days and Laverne And Shirley had characters appearing on each other's shows fairly regularly, and there was one true crossover storyline that spanned episodes of both series — which was only reasonable, since Laverne And Shirley was a Spin-Off of Happy Days. And just to add a little weirdness to the mix, Mork and Mindy was also a Happy Days spinoff...
The Golden Girls, Empty Nest, and Nurses were all made by the same production company and all shared the same Miami setting, so there was predictably a fair amount of crossover among them.
And for another loop when the appearance of Bill Smitrovich as a guest star on Boston Legal, in a different role from the cranky, vindictive D.A. he played on The Practice.
The role was Not So Different at all. The main difference was that his Boston Legal character worked in a state where Death Penalty was applicable.
The most baffling loop-thrower has to be John Laroquette, who had a notable guest appearance as a devious client on The Practice before showing up as regular character Carl Sack in Boston Legal's fourth season.
Friends crossed over with The Single Guy, as well as with Mad About You. (A minor character on the latter show, Ursula the waitress, is actually the sort-of-evil twin sister of Friends' Phoebe.)
Also, there was a time when Chandler knew Annie Spadaro, of Caroline In The City, that is too a cameo of the former in this series.
There hasn't been a direct crossover between The Shield and Sons of Anarchy, but a gang called the One-Niners, based in Los Angeles, has appeared in both shows, indicating that the shows share a continuity. Considering that one of Sons Of Anarchy's producers was a writer on The Shield, it's not surprising.
All of the series in the DCLAU tend to have numerous crossovers with eachother. In 2006, That's So Raven, Hannah Montana, and The Suite Life of Zack and Cody have crossed over (in a special called That's So Suite Life Of Hannah Montana), and, in 2009, there was an another one with Hannah Montana, The Suite Life On Deck, and Wizards of Waverly Place (Wizards On Deck With Hannah Montana). Interestingly enough for shows with little enough continuity, the characters from The Suite Life and Hannah Montana recalled the previous crossover.
In the realm of examples that don't inflict pain in the viewing audience, from Lost Galaxy on, there's a recurring tradition of the seasonal team-up, a two-parter where the previous season's cast meets up with the current Rangers to deal with a threat that requires their combined powers.
There was also a three-part episode of MMPR that featured the main character of of the Masked Rider series. Interestingly enough, 13 years later...
Kamen Rider Decade, itself an anniversary crossover of the previous 9 Kamen Rider titles, became the first Kamen Rider series to cross over with a Super Sentai series, with Tsukasa and Daiki appearing in two episodes of Samurai Sentai Shinkenger, and the Shinkenger appearing in an episode of Decade. This, in turn, was followed by...
Kaizoku Sentai Gokaiger, the 35th Anniversary Super Sentai series, features characters who can use the powers of the 34 previous teams, and at least one member of most of the previous teams have appeared on the show.
Ultraman and Kamen Rider had a crossover short in the early 90's, unexpected considering each series is from a different company.
Ultraman Mebius has crossovers with older/the original Ultra Series throughout its run. Be it from classic monsters to the older heroes showing up for a team up. The Movie, Ultraman Mebius and the Ultra Brothers takes this to 11.
The Polish Crime and Punishment SeriesW-11 and Detektywi made at least one such crossover: an episode on Detektywi was the first part, depicting the eponymous detectives investigating a case; then, for the second part, the policemen of W-11 took over.
EUReKA and Warehouse 13 have a pair of crossover episodes, where Fargo upgrades the Warehouse's computer system and Claudia visits Eureka. Fargo even managed to MacGyver a lightsaber with a laser pointer and an artifact. SyFy even used these episodes for a canon crossover ship. Claudia/Fargo. Alphas was also given a crossover with Warehouse 13, although no character in Alphas has yet appeared in either of the others.
In The Man from U.N.C.L.E.'s heyday an episode of Please Don't Eat The Daisies had the children believing their father was a spy after seeing him pass a matchbook to Illya Kuryakin in an innocuous (or was it?) encounter. Hilarity ensues until, in the show's concluding scene, someone is brought in to convince the children that they've jumped to a foolish conclusion. It is Robert Vaughn whom the kids recognize instantly as Napoleon Solo.
Word Of God says that if Psych hadn't made Leverage a fictional show within Psychs universe, Leverages Eliot would have had an uncle named Henry.
The WB once had a night dedicated to crossing over their shows. A character from one show would pop up in another, mostly for laughs and night contribute much to episode's plot. It was parodied in that night's episode of Unhappily Ever After, wherein Jackee Harry and hinted that The Wayans Bros. were "waiting in the car".
Breakout Kings : In the third episode of the first season, Prison Break's T-Bag....breaks out from a prison. An unprecedented case of a character crossing over to a show on a completely different network but not so surprising considering both show have the same producers (and an obvious common theme).
The Six Million Dollar Man and The Bionic Woman had regular crossovers, with two-part and three-part stories involving both shows (most notably "Kill Oscar," which had parts 1 and 3 on Jaime Sommers's show with the middle part on Steve Austin's). Curiously, although both series aired on ITV in Britain, the crossover episodes weren't treated as such: all three parts of "Kill Oscar" and both parts of "Welcome Home, Jaime" were aired as episodes of The Bionic Woman, while both parts of "The Return Of Bigfoot" were shown via The Six Million Dollar Man.
A 1968 Thanksgiving episode of The Beverly Hillbillies had that show's characters traveling to Hooterville, leading to a three-way crossover between The Beverly Hillbillies, Petticoat Junction and Green Acres. Earlier episodes of Green Acres had shown Hootervillians watching the fictional Clampetts on TV and even producing an amateur play based on The Beverly Hillbillies TV show.
USA Network had a set of commercials crossing over their various shows; Adrian Monk and Johnny Smith are each thankful the other doesn't want to shake hands; Johnny and Shawn Spencer debate who has the worse past; until they notice Monk counting his food; etc.
And Bud Roberts from NCIS' parent series JAG appeared in one episode in the first season.
And then NCIS Los Angeles and Hawaii Five-0 did a two-way crossover where Callen and Sam came to Hawaii to prevent a black-market bioweapons buy, then Danno and Chin Ho went to L.A. to stop one of the involved parties from unleashing the bioweapon in question.
Naturally, Buffy the Vampire Slayer and Angel did it several times. Except during season 6/3, when Buffy's Channel Hop made it a lot harder and they were essentially banned. Then the ban was lifted, and crossovers happened again.
The Fox show The Finder, besides having been given a Back Door Pilot from Bones, later had a crossover from the same when Sweets popped up to do a psychological evaluation of Walter. Dr. Jack Hodgins appeared in an episode of The Finder.
Tinsel was crossed over by Jacob's Cross during the brief engagement between Phillip Ade-Williams and Chi-Chi Nwachukwu-Abayomi.
Shonda Rhimes' Grey's Anatomy and Private Practice have crossovers seemingly every five episodes or so. Now that Rhimes has developed a new show, Scandal, one wonders just what kind of menage a trois is in store.
8 Out of 10 Cats does Countdown (twice) and Deal or No Deal (once — and it's the first DoND UK episode not hosted by Noel Edmonds).
ER crossed over with Third Watch. Unsurprising, as they were both created by John Wells and have similar subject matter. Unfortunately, it ended up negating a throwaway line in which a Third Watch character mentioned watchingER on television (obviously a Shout Out, but it would mean that the two shows didn't have a Shared Universe).
Older Than Feudalism example: the classical tale of Jason and the Argonauts brought together many mythological heroes in a sort of Justice League Ancient Greece. "Hero" in Classical Mythology was an awfully vague term (as Larry Gonick put it, heroes aren't necessarily heroic, just excessive), and since the mission was to steal an item (the Golden Fleece) "Justice League" isn't a terribly good name for them....
Maid Marian, the shepherdess, featured in many pastoral plays popular at May festivities. Then there was a fad for Robin Hood plays. Then someone put the two characters together. It worked so well that Maid Marian is best known as a Robin Hood character, even though her independent existence predates him.
Morgan Le Fey originally appeared in Chivalric Romance in the Matter of France — tales of Charlemagne — not the Matter of Britain — tales of King Arthur.
Anything that can be deemed an "interpromotional match" can be considered a crossover of one talent for another promotion.
After the WWE brand split, any match that had a Raw talent going up against a Smack Down talent was considered this, and thus, you got a good bit of cross overs between the two brands. This was further jettisoned with the "Supershows", where Raw and Smack Down stars were on the same shows (to the point where from August of 2011 to the Raw 1000th episode, Raw was renamed the "Raw Supershow" with the word "supershow" being tacked on to the Raw logo in the style of the Smack Down logo
Perhaps the most well known crossover in wrestling happened on the last episode of WCW Monday Nitro. After Vince Mc Mahon's purchase of the promotion, a simulcast was scheduled, with Vince on WWE Monday Night Raw bragging about how he bought his competition, while Nitro featured the WCW talent wrestling for perhaps their last time on television. The final minutes, however, proved to be the most memorable. On Nitro, Shane Mc Mahon made an appearance on Nitro (in Panama City, FL), while Vince was at the Raw location (in Cleveland, OH). The titantron showed Shane on Raw, while Nitro had its Nitrovision show Vince. The segment was to begin the invasion storyline (and
that, in kayfabe, that Shane was the actual owner). It was memorable in that both TNT and TNN were basically broadcasting the same thing during those final ten minutes or so, but in different settings.
During the ECW invasion storyline in 1997, Jerry Lawler appeared on ECW programming despite being contracted by the WWE. It was actually a work, as Vince had actually been attempting to help ECW be recognized, with Paul Heyman making several appearances on Raw during the angle.
On the April 1, 2013, broadcast of "Family Life Today", John Fuller and Jim Daly, from "Focus on the Family", crossed over.
An arguable one, similar to the Comic ReliefPanel Game crossovers above, the New Year 2010 episode of The BBC Radio 4 comedy panel game The Unbelievable Truth featured Stephen Fry, Alan Davies, regular QI panelist Rob Brydon and QI producer John Lloyd. The format of the game was unchanged, but the "obvious answer" klaxon was added. Much to the satisfaction of the other players (and host David Mitchell), Stephen set it off twice.
The Archers had a crossover with Gardener's Question Time for Easter 2011, where the real-life panel, chaired by the real-life Eric Robson answered questions from the population of Ambridge.
Two episodes of The Goon Show featured guest appearances by Jack Train as Colonel Chinstrap from ITMA, a popular comedy show from the previous decade.
"Reality Storm: When Worlds Collide" crossed over Hero Games' Champions and Guardians Of Order's Silver Age Sentinels, co-produced by both companies. The adventure itself was dual-statted, but the book also included official conversion notes between the Tri-Stat d10 system and 5th edition Hero System.
The Spelljammer novels of The Cloakmaster Cycle demonstrated such a campaign, with most of the first book taking place on Krynn, only gradually introducing the concept of spelljamming to the hero. Later he visited two of less known locations on Toril. At least Forgotten Realms actively supported the meeting with other worlds by dropping mentions in its own sources.
West End Games' Paranoia had a trilogy of crossovers under the banner "Vulture Warriors of Dimension X". The first two adventures crossed over with Cyberpunk 2013 and Twilight2000, while the third adventure used time- and dimension-travel as a campaign hook for open-ended crossovers with other franchises.
A more recent example (1829) is the theatrical play Don Juan und Faust by Christian Dietrich Grabbe, a German dramatist. It is a crossover of the myth of Faust and of the tale of Don Juan.
Forza Motorsport 4 features the Top Gear (UK) test track, and Jeremy Clarkson will provide commentary on dozens of cars in the Autovista mode. All the Reasonably Priced Cars from the UK and US Top Gear will show up as well.
The Warthog from Halo will appear as an Easter Egg in the Autovista mode with Cortana (Jen Taylor) providing the commentary for it in the place of Clarkson. Unfortunately, it's not drivable.
AdventureQuest Worlds itself is one huge mix of the three main AE games, Adventure Quest, Dragon Fable, and Mech Quest. Heroes and villains from all three have been featured in the game, although arguably the most evil villain of all three gets killed off right in the beginning of the game.
Before that, DragonFable and AdventureQuest had a crossover mission.
Also, AdventureQuest and Warp Force are in the same period of time, but it began with a crossover from AdventureQuest.
One game in its 3D spinoff Maximum Impact (Mainly MI:2) had Hanzo Hattori from Samurai Shodown and Fio Germi from Metal Slug as unlockable characters.
Metal Slug 6 and 7 featured Ralf Jones and Clark Still (originally from Ikari Warriors, but using their appearances from the KOF franchise) as playable characters, with fellow Team Ikari member Leona appearing as a DLC character in Metal Slug XX, a PSP re-release of Metal Slug 7.
In addition to some already present in the KOF series, Neo Geo Battle Coliseum also added characters from other SNK titles that the KOF series didn't add, as well as some ADK titles (due to SNK Playmore's purchase of ADK after it filed for bankruptcy).
The Rival Schools games have had a few crossovers with Capcom's more popular fighting game series Street Fighter; the most obvious was Sakura Kasugano, Ryu's Hero Worshipper, being a playable character in the console versions of the game. There is also a character known as Ran Hibiki, whose name is almost exactly the same as that of Dan Hibiki. Just what her relationship to him is undetermined, assuming there is a relationship at all and it is not simply an homage.
Hinata also claims to be a student of "Masters-ryu Karate" in Nekketsu Seishun Nikki 2, which explains why some of her special moves have the same flame effect as Ken's Dragon Punch in Super Street Fighter II.
Also in Street Fighter, Cody, Guy, Rolento, and Sodom from Final Fight appear as playable characters in the Alpha series. The crossover between the two also carried over into the cartoon, which featured a retelling of Final Fight's storyline while adding Ryu and Ken.
There are stories and videogames which also cross him over with the Cthulhu Mythos.
The Sega Superstars series subverts this at first, as it is essentially a collection of games based on Sega properties, but the second game, Superstars Tennis has Sega characters all play tennis together, playing the trope straight. Then there was a third racing game, which was a lot better than expected.
Before that we had, Fighters Megamix, which included most of Sega's 3D characters in a fighting game.
While it makes sense since they are a part of the same cosmology, most of Bioware and Obsidian's D&D-based games have subtle crossovers. Most notably, Baldur's Gate II features artifacts from some of the characters of Planescape: Torment, and one of said characters is also mentioned by an NPC in Neverwinter Nights 2.
Battletoads and Double Dragon: The Ultimate Team was one of the earliest intercompany video game crossovers, which was made possible by the fact that both series were published in America and Europe by Tradewest. The game itself is really a Battletoads game with Double Dragon characters as guest stars (some of which were misnamed).
Cross Edge is this in and out, with characters from the Disgaea, Darkstalkers and Atelier franchises making playable appearances (some, like Marie from Atelier Marie, being shown to international audiences for the very first time).
Dissidia: Final Fantasy. It's the main heroes and villains of Final Fantasies I-XII beating the ever-loving crap out of each other!
The retail version of the DoDonPachi DaiFukkatsu Black Label 360 port features an arrange mode that crosses DoDonPachi over with Ketsui, with scoring elements, player ship, and True Final Boss taken right out of Ketsui.
Half-Life and Portal were originally different universes altogether. (GLaDOS vaguely alludes to the events of Half-Life once in passing, but that was just an easter egg.) But then! In Portal 2 you can find the drydock for a ship called the Borealis, and Half-Life 2: Episode 2 ends with Gordon Freeman about to set off to find it.
The shared universe is not only an allusion in the first game - in several rooms the player can find projectors with slides explaining explicitly that Black Mesa is a (hated!) competitor of Aperture Science.
While we're at it, the first official mod for Skyrim, after the long-awaited HD Texture Pack... was a collaborative effort between Valve and Bethesda, showing just where that adorable littleSpace Core wound up.
And one of the fans wrote a mod that adds a crashed SSV Normandy from Mass Effect.
Blockland has a ROBLO Xian add-on that can make it be this
PAYDAY The Heist, with its gameplay so inspired by Left 4 Dead, went the full mile and released a heist based on Mercy Hospital from L4D's "No Mercy" campaign.
Pop 'n Pop, an obscure puzzle game does this with both the gameplay and the characters. The gameplay is essentially Puzzle Bobble with a few elements from Space Invaders thrown in while the roster is made up of characters from various Taito arcade games.
Capcom and Valve are coming together to have a crossover of sorts in each other's games. For Valve, they are bringing Rochelle, Coach, Ellis and Nick (the survivor characters) to Resident Evil 6 on the PC in Mercenaries mode. For Capcom, they are bringing the Lepotitsa, Napad and Ogroman as special infected counterparts to Valve's special infected for Left 4 Dead 2.
Way before that, thanks to players modding the game, you can have various characters from many other games be playable within Left 4 Dead and Left 4 Dead 2.
Even the venerable Mario Kart series does this... although most of the characters are Marioverse stalwarts, Donkey Kong and Diddy Kong are their Donkey Kong Country incarnations. Being co-developed by Namco, the Mario Kart GP arcade games go further than the home console versions and include characters from Pac-Man.
Despite being an internally consistent multiverse, the Nasuverse only has crossovers in incredibly round-about ways. Part of the problem is, well, Word Of God stated that some of the characters simply cannot exist in the same universe for Canon reasons.
Aoko Aozaki spends time in the beginning of Tsukihime with a young Shiki Tohno. Her sister, Tohko Aozaki, is the Older and Wiser of (arguably) Tsukihime-prototype, Kara no Kyoukai. Despite each of them knowing a different Shiki (each possessing the Mystic Eyes) it's explicitly stated that Tohko and Arcueid cannot exist in the same universe (and most likely, Shiki Ryougi and Shiki Tohno can't, either).
It's more along the lines of "Tohko's goal contradicts with the existence of True Ancestor Arcueid", though Nasu hasn't really made it clear on why. And Mystic Eyes are just... eyes with special powers. Many, many characters have them, just that there can't be more than one possessor of the Eyes of Death Perception.
Shiki's Eyes of Death Perception is different from the genetic eyes of the Nanaya. Since both Shikis are from closely associated families of assassin though, it might be related to genetic. However Ryougi's family does not possess special eyes. The reason why only one can exist might be due to the nature of those eyes. Given that they are potentially insanely powerful and you can't just have people running around with such things, it might be a case of The Chosen One.
It's just chance. The Eyes of Death Perception have a rare chance of showing up in their respective families. It's not that they can't exist in the same universe, it's that the chances are very, very low.
Sadly, the long-awaited battle between Black Mage and Richard the Warlock never happened since they were apparently on the same side, but 's not to say it never will. You never know with Sohmer and Lar.
Props if you noticed the background of the first 8-Bit Theater in Rayne and John's apartment window on one of the pages.
One of the earliest webcomic crossovers was a week of strips in 1998 between User Friendly and Sluggy Freelance where Torg temps at the ISP while Riff ends up battling the Coffee Monster.
Emergency Exit is quite famous for its very extensive crossover with Parallel Dementia, notably having a drastic effect on the plots of both series. It was also very, very long. Emergency Exit also had a crossover with Beyond Reality before that (Though that crossover didn't really effect the plot at all) and later had a second crossover with Parallel Dementia that consisted largely of Fall and Eddie having a conversation, with several big revelations made.
Jix has at least two crossovers from the creator's other series Dragon City. Both of which involve stories told about encounters with dragons, both of which were also mentioned in Dragon City, but without the Jix characters. Both times it involved main characters.
Also, in Dragon City, there were four major crossovers from Jix. One was when an Ambis (the alien species from Jix) tried to take over the dragons' world, another instance when one of the villains tried to kill Erin because he thought the previous crossover was still disguised as her, one was when Jonas got kidnapped by one of Jix's minor villains, and the last was when Lauren and Jix saved Erin from her editor.
In summer 2010, the world was introduced to an Axe Cop and Dr. McNinja crossover. And it was awesome.
Fans! had a crossover with Penny and Aggie recently, or at least the version of Penny and Aggie that exists in the Fans-verse. Charlotte turns to dark magic during the story and after being taken into custody by Aegis, ends up reluctantly helping them in their fight against The Order.
Deadlands is a game combining two unconnected categories - Western and Horror. As a result of the popularity of this refreshing juxtaposition, we can expect a slew of new games created by stitching together various "best selling" categories. Here's a preview...
There's a crossover saga in Slimy Thief where in each arc the main character, Aisha, interacts with other creator owned characters. Apparently portals are opening up in alternate dimensions and sucking characters into hers.
There was a multi-episode arc of Space Ghost where the title character went up against the Council of Doom, each of whom managed to defeat Space Ghost and banish him to some other realm/time/planet, just so the other Hanna-Barbera heroes of that setting could help save him. This allowed Space Ghost to team up with the Herculoids, Mightor, Shazzan, and Super Moby Dick.
The 1981-82 Hanna-Barbera show Space Stars was also built around this trope. Each hourlong episode would have a short eight- to 10-minute segment starring one of the show's hero teams: Space Ghost, The Herculoids, the Teen Force, and Astro from The Jetsons. The last five to 10 minutes of the show would then be a "Space Stars Finale" team-up between two or more of the groups against a common foe.
[Homer and Bart accidentally pick up Bender of Futurama while driving through a portal tunnel]
Bender:Oh boy! You guys are gonna be my best friends, right?
Homer:You wish, loser!
[Throws Bender out the car.]
Then there was the time The Simpsons crossed over with The Critic, which Matt Groening was against due to his hatred for The Critic and his promise at the time that his cartoon series would be different from other sitcoms (read: it wouldn't do cheap crossovers for ratings). Because of this, Matt Groening's name is not shown in the credits to "A Star is Burns" (making it the only instance that this has happened). Ironically, Groening's name appears in the credits of the season eight episodes "The Springfield Files" (which is also a crossover episode, only with Mulder and Scully from The X-Files) and "Hurricane Neddy" (which featured a brief scene of Jay Sherman in a mental hospital room repeatedly spouting his catch phrase "It stinks!", with a doctor trying to calm him down), as well as the episodes that briefly crossed over with Futurama and the season 18 episode "24 Minutes" (which is a crossover with the show24).
Spider-Man: The Animated Series had a crossover with the 90's X-Men animated series. It was considered a big deal because it was a completely different animation studio involving the then current roster from X-Men in a show that was not their own. Even more impressive was the effort put in to keeping all the same cast (save for one, Gambit, presumably for contractual or scheduling reasons) for the sake of continuity. Even more fun, the crossover remains in continuity for Spider-Man, as Storm returns during the series' adaptation of the Secret Wars crossover event.
The Marvel cartoons from around this era were frequently cameo-ing in each other's series, as well (though it's hard to know if they were the same characters as the other cartoons; they all take place in aMarvel Universe, where a Spider-Man, Human Torch, etc. would likely exist somewhere.) You never know who'll be briefly shown watching from a rooftop, or looking up at the Pillar of Light in the distance when something really big goes down. Also, Iron Man, Fantastic Four, Incredible Hulk, etc. guest starred in each other's shows often enough to make the 90s Marvel cartoons a Diniverse of sorts - you can connect the dots through all of them.
It was a bit more subtle than most of these examples, but G.I. Joe crossed over with The Transformers in the episode "Only Human". A masked character named "Old Snake" (voiced by Chris Latta) helps the Big Bad of the week with a machine to put Transformers' minds into synthetic human bodies. After the plan inevitably fails, Old Snake escapes and idly muses that "they just don't make terrorists like they used to", then raises his arms and yells "COBRAAAAAA!", ending in a coughing fit.
At the time the episode aired, it initially had a hard time fitting in with the events of GI Joe The Movie, where Cobra Commander was transformed into an humanoid yellow cobra-man and then finally doomed to a fate as an actual non-anthropomorphic cobra. However in "Only Human", Cobra Commander is obviously humanoid and visible from his torn-up gloves are hands covered in yellow scales; presumably in reference to when he was transformed into a yellow-scaled snake man in the movie. Through what was likely coincidence, this seemingly erroneous depiction of a future Cobra Commander was later made to make sense: In 1989 (three years after "Only Human" originally aired), DiC produced a continuation of the G.I. Joe animated series, beginning with the five-part mini-series "Operation: Dragonfire". In this mini-series, Cobra Commander is still just a snake, but he's eventually returned into a humanoid form (specifically the snake-man form) by the Baroness.
And from the same season, recurring character Marissa Faireborne was obviously the daughter of Flint (Dashiell R. Faireborne) and Lady Jaye. The DVD commentary for Transformers The Movie finally admitted this officially.
One "cameo" was made when a hologram version of an aged Flint was used in order to trick Marissa in one episode.
In a more meta case, all of Marvel/Sunbow's cartoons of The Eighties (Transformers, G.I. Joe, Inhumanoids, Jem etc.) were tied into each other via Hector Ramirez, an Expy of television reporter Geraldo Rivera. Hector would show up just about any time a story needed a television reporter, suggesting the shows all resided in a shared universe.
There's one line of promos they did called "Perfect as It Is", which show that certain cartoon character cross pairings just don't work out right. Take a look at "Tom and Speedy" (there was also "Road Runner & Dexter" and "Batman and Inch High Private Eye".
A crossover of Darkwing Duck with Chip N Dale Rescue Rangers was planned but was never produced beyond a few voices. A quick clip of these voices can be heard in the Darkwing episode "Twitching Channels".
Darkwing Duck did have DuckTales' Gizmoduck as a reoccurring character though.
And in one episode, the entire recurring villain cast from DuckTales showed up in one scene.
Animaniacs sometimes did crossovers with other cartoons on its show, including one incident where Batman rescued them from an angry fairy (as Yakko's Shakespeare recital had foretold). Coincidentally, Batman The Animated Series aired right afterward.
One Animaniacs episode featured internal crossovers, with Dot and Slappy Squirrel trading places, as well as shorts featuring Mindy and the Brain, Pinky and the Cat (Rita), and Pesto and Runt.
Buddy, the failed successor to Bosko, appeared in The Warners 65th Anniversary Special.
Superman and Batman (in their Bruce Timm/Paul Dini incarnations) crossed over multiple times, starting with the World's Finest three-parter. Eventually, this led to the DCAU, which had this trope as its foundation.
Interestingly, MAD did the very same crossover earlier.
Also the "Imaginationland" trilogy could also be considered something of a massive crossover as well.
Bart Simpson of The Simpsons appears in the two-part "Cartoon Wars". There are even references to his series, including mentioning that he stole the head off a statue once (a reference to the episode "The Telltale Head" and him saying "Cowabunga!" (in The Simpsons episode "Behind the Laughter", Bart, after a sketch written by Homer in which Bart has the line "Cowabunga!", remarks with irritation that he's never said "Cowabunga" in his life).
Ace Ventura and The Mask crossed over on two episodes: the final episode of The Mask ("The Aceman Cometh") and the last episode of Ace Ventura's second season "Have Mask, Will Travel."
Back in 1972, The ABC Saturday Superstar Movie anthology series featured Looney Tunes characters in a Filmation production, Daffy Duck and Porky PigMeet The Groovie Goolies — an atypical crossover of characters from different animation studios. (Consider that veteran WB animators, such as Ted Bonnicksen, Virgil Ross, and Norm McCabe were all working at Filmation at the time.)
Another King Features example: The main characters of Defenders Of The Earth (King's action/adventure heroes Flash Gordon, The Phantom and Mandrake the Magician) were all from previous series, making the entire series a Crossover.
Seth MacFarlane's Family Guy, American Dad, and The Cleveland Show have had characters make occasional appearances, but the first true crossover among them is Night of the Hurricane, in which Hurricane Flozell affects all three shows and the actual "crossing over" occurs after the hurricane when Cleveland Brown, Stan Smith and Peter Griffin get involved in a stand-off.
An episode of The Zeta Project crossed over with Batman Beyond, justified in this case as the former was a spinoff of the latter. (Due to Bob Kane's contractual billing being what it is, this is also the only episode where the opening titles omit the "Created by Robert Goodman" credit - the end credits specify Kane's being behind Batman, with Goodman being behind the characters for the spinoff.)
He-Man and She-Ra did this more than once, as special episodes. Made sense, since they were siblings, but every time it happened it was a big deal. The two would do their transformation sequences simultaneously, making for a doubly psych-up scene. Strangelynote or not, if you're a cheap scene-recycling animation house though, despite being in the same place at the time, they would then each appear in front of their respective castles, which were located in different countries.
Eek! The Cat, it would seem, shares a universe with The X-Files; the episode "Eek Space 9" features cameos by Mulder and Scully (several years before The Simpsons) - an alien ship crashes into Scully's office and she screams that he was right all along (a not inaccurate prediction of how her character turned out on the live action show}!
the show Houseof Mouse was all about Disney animated characters going the house to watch various acts.
'Victor & Hugo - Bunglers in Crime' - Many times, either in reference or in guest stars. Count Duckula, Igor, Nanny, Hawkeye Soames and Dr. Potson (from 'Count Duckula') Danger Mouse, Colonel K, Baron Greenback, Nero and Stiletto (from Danger Mouse) and even a nod to Badger from 'Wind in the Willows. It seems fitting since 'Victor & Hugo' were a spin-off from 'Count Duckula' who in turn span off from 'Danger Mouse.'