Continuity Overlap

Events in one series also impact continuity in another series. This an element that may go into defining a 'Verse.

Immensely common in American comic books, to the point where some series essentially merged into one during the '90s.

See also Crossover, Intercontinuity Crossover, and Massive Multiplayer Crossover.

Anime/Manga

 * In Digimon, after the events of Anode Tamer and Cathode Tamer for the Wonderswan - sorry, No Export for You - Ryo Akiyama dimension-hops to the Adventure universe. He witnesses the battle with Diablomon from the second movie and meets with Ken Ichijouji. Ryo and Ken defeated Milleniummon in Tag Tamers and Ken is infected with a Dark Spore, setting events in motion for Adventure 02 and confusing the hell out of anyone who had to take Ken's flashbacks to those events at face value with no knowledge of the games. Later in Brave Tamer, his partner Monodramon merges with Milleniummon resulting in Cyberdramon. Ryo then appears in Tamers, a universe separate from Adventure, and he has Cyberdramon with him who is aggressive and uncontrollable due to Milleniummon's half of his being. So this is basically three (or more) very separate continuities all connected through Ryo.
 * It should be noted that Ryo's movie and 02 scenes go unnoticed by American viewers because he resembles Wonderswan Ryo far more than Tamers Ryo because of the art style (even once Tamers Ryo leaves the Digital World and gets the shirt he has in Anode/Cathode.) Once you know what he looks like, there's no mistaking his movie and 02 flashback appearances for anyone else.
 * All of Ken Akamatsu's manga series seem to have this, especially Love Hina and Mahou Sensei Negima. Specifically, both Love Hina and Negima have identical characters named Mei, who appear to be the same person, Word of God states that the inn that Keitaro and Naru stayed at in Love Hina was the same inn that several characters from Negima stayed at, and the Shinmeiryu sword school is used by characters in both series. In a recent Negima chapter, Motoko's family name was explicitly mentioned in relation to Shinmeiryu techniques, and a specific technique ("Zanmaken: Ni No Tachi") used in Love Hina has minor plot relevance in Negima.
 * Negima also has a shout out to his first manga, A.I. Love You, when the subject of sentient A.I. comes up; it's hinted that part of Chachamaru's programming may have been written by A.I. Love You's protagonist.
 * Akamatsu himself has confirmed that the Nitta-sensei from A.I. Love You and the Nitta-sensei from Negima are the same person, as well as the fact that Motoko and Eishun are related somehow.
 * XXX Holic and Tsubasa Reservoir Chronicle both merge at numerous times, and characters in one series are often seen in another. The anime for both series, due to unfortunate copyright issues, doesn't cross over as much.

Comics

 * Clearly the Marvel Universe, which includes characters from the Fantastic Four, The Incredible Hulk, Spider-Man, Iron Man, Captain America, Ghost Rider, The X Men, The Punisher and many more.
 * A limited amount of this has appeared in the recent Marvel movies -- Nick Fury appears at the end of Iron Man, and Tony Stark appears at the end of The Incredible Hulk. This crossover has, so far, not appeared in the Spider-Man or X-Men movies.
 * The lack of crossover with the Spider-Man and X-Men franchises are due to the film rights to those characters being owned by Sony and Fox, respectively, as opposed to Marvel Studios.
 * Behind-the-scenes pictures of some of the maintenance panels on the X-Jet in X2 reveal that some systems were built by Stark Enterprises (and that there were no user serviceable parts inside!), but they're far too small to be seen onscreen. An "Intercontinuity Nod"?
 * DC Comics has done this as well, notably with Batman and Superman.
 * It's not uncommon for a main character in one Sin City story play a minor role or even a cameo in a different story. Many stories have also been shown to happen at the same time, such as Hard Goodbye and A Dame To Kill For.

Film

 * The Marvel Cinematic Universe is one of the very rare movie examples; while all movies are self-contained, there are lots and lots of references to the other ones. Nick Fury has appeared in Iron Man, Iron Man 2, Thor and Captain America: The First Avenger, while Tony Stark made an appearance at the end of The Incredible Hulk. In The Avengers, the main characters from all the mentioned movies will join forces. Sadly, we're Screwed By the Lawyers when it comes to anyone from the Spidey or X-Men films appearing, despite Wolverine and Spider-Man having been in Avengers comics for some time now and Beast having a long history with them.
 * The View Askew Niverse, The Verse in which Kevin Smith has set 6 films, several comic books, and an Animated Adaptation.
 * Several characters will be heard about in one movie, only to be seen in a later film.

Literature

 * In the Honorverse the novels Ashes of Victory/Mission of Honor, Crown of Slaves/Torch of Freedom and Shadow of Saganami/Storm from the Shadows all take place concurrently, so we get three different looks at the same sequence of events, and often have entire chapters duplicated across books.
 * In the Dragonriders of Pern novels, the events of Dragonquest, the second Lessa novel, affect the events of Dragonsinger, the second Mellony novel.

Live Action TV

 * Star Trek the Original Series did this as well when Deep Space Nine, Voyager, and The Next Generation films were all running at the same time.
 * Miles O'Brien passed over to Deep Space Nine in its pilot episode. After the destruction of the Enterprise in the Generations movie, Worf joined the station as well. The Deep Space Nine crew changed uniforms after First Contact.
 * There is an entire episode of Voyager where the destruction of the Maquis by the Dominion/Cardassian alliance over in Deep Space Nine was a key part of the plot.
 * Buffy the Vampire Slayer and Angel interacted so much during their respective fourth and first seasons that watching them in concert is almost a neccesity to understand what's going on in either.
 * There is Continuity Overlap between the last seasons of Stargate SG 1 and the start of Stargate Atlantis, as well as between the end of the fourth season of Stargate Atlantis and the Stargate Continuum movie.
 * The new series of Doctor Who, The Sarah Jane Adventures and Torchwood have the occasional continuation between them. For example, the Sontaran invasion in Doctor Who leads to a stranded Sontaran soldier in The Sarah Jane Adventures.
 * Bryan Fuller's works -- Dead Like Me, Pushing Daisies and Wonderfalls -- are all apparently set in the same universe. Pushing Daisies and Wonderfalls share a character (a one-timer) and Pushing Daisies also refers to Happy Time Temp Agency, which is where George worked in Dead Like Me.
 * Friends, Mad About You and Caroline in The City are all in the same universe, although they don't have a major impact on one another, just cameos by characters in each work.
 * CSI hosted the Poorly Disguised Pilot of CSI: Miami, which in turn CSI New York launched from, so the teams exist in the same universe, and even a killer that was featured first in Miami was caught in the NY show.
 * Similary, JAG spawned NCIS which spawned NCIS Los Angeles. Other than the Poorly Disguised Pilot, there was one minor cross-over of a JAG lawyer appearing on NCIS.
 * And to this troper's knowledge, Abby Sciuto is the only character on either of the three to appear on all three shows.
 * The entire Law and Order universe, with guest appearances by many actors on other shows.
 * Homicide Life On the Street had a few cross over episodes with Law & Order.
 * Detective John Munch (Richard Belzer) started out on Homicide -- and has also played the character in no less than eight different shows, for at least four different networks.
 * The Sy Fy Channel's played this game, with crossovers between Eureka, Warehouse 13, and Alphas.
 * When Bobby Ewing was killed off on Dallas, it was mentioned in Spin Off Knots Landing, and Gary Ewing named his newborn son after his late brother. After the infamous All Just a Dream Reset Button that brought Bobby back, Knots Landing stopped referencing its parent show.

Web Comics

 * Megatokyo's Junpei has shown up in Apple Geeks, and they cross over from time to time. Mostly though, they stay apart, mainly because one set of characters in in Japan and the other is in America. Plus, time passes much slower in Megatokyo than in Applegeeks.

Web Original

 * Almost all of the Whateley Universe stories take place among a small group of kids who go to Whateley Academy together, so there's phenomenal amounts of Continuity Overlap, to the point that it looks like the authors have to coordinate character schedules down to the day.

Video Games

 * All of Volition's games take place in the same continuity, where the Saints Row series serves as a prequel to the Red Faction series, taking place Twenty Minutes Into the Future.
 * Valve's Half-Life and Portal videogame series canonically exist in the same universe.
 * Portal was introduced in the "Orange Box" compilation as a Gaiden Game borrowing elements of the Half-Life universe -- specifically, it mentions Black Mesa as a competitor to Aperture Science for government funding, and Word of God says that GLaDOS awakening and subsequently going berserk coincided with the Combine invasion of Earth, preventing the scientists from getting help.
 * Half-Life 2: Episode 2, for its part, introduces a ship called the Borealis as a major plot element; said ship was built by Aperture as part of an early teleportation experiment and "vanished, along with part of the dry-dock".
 * Portal 2 continues the gag with an Easter Egg where you actually find the other part of the dry-dock, complete with a life preserver labeled "Borealis", and Cave Johnson directly mentions Black Mesa in the pre-recorded messages you find in Old Aperture.