CSIVerse has been known to cross storylines and characters between its various incarnations, as has the Law & Order series. This most commonly takes place during sweeps.
The most prominent CSI example is their "trilogy" from Nov of 2009, which concerned a nation-wide human trafficking ring Ray Langston was after due to young woman being kidnapped. It starts on CSI: Miami with "Bone Voyage," travels up the East Coast to CSI: NY with "Hammer Down," and wraps up in Vegas with CSI's "The Lost Girls."
There was in fact a Law & Order massive crossover in the works, involving a terrorist plot to attack NYC and several teams of detectives from different squads all working the case together. The idea got shelved after 9/11.
During the winter of 1999, in line with its plans to air the MiniseriesStorm of the Century, ABC had a raging winter storm strike all four of its soap operas. Fitting, as two were set in upstate Pennsylvania, while the other two (one of which was a Spin-Off of the other) was set in upstate New York.
The CW's Arrowverse has been doing these ever since The Flash premiered; it's not uncommon for characters to visit each other, but the ones that air each fall just before the winter break are advertised as big event TV. Originally they were just with the parent show, Arrow, they've gotten bigger each year as more and more shows are added to the franchise. They are in order:
The 2014 event, Flash vs. Arrow, where Flash and Arrow face off and visit each other's cities to understand each other better.
The 2016 event, Invasion!, an adaption of the Invasion! arc of the 1980's expanded the crossover by having play across all four shows: Supergirl, The Flash, Arrow, and Legends of Tomorrow by having Barry Allen unite all the heroes he knows to stop an impending invasion from an alien race called the Dominators.
The 2017 event, Crisis on Earth-X is even bigger. Whereas Invasion! was technically only a three part event (with Supergirl being just a tie-in to the main event), this crossover fully plays out over all four episodes, equally juggling the storylines of each show, as well as serving as a lead-in to Freedom Fighters: The Ray. And it actually has lasting repercussions for the Arrowverse's future, as Barry and Iris get married, as do Oliver and Felicity, while Martin Stein is Killed Off for Real.
The 2018 event, Elseworlds, dials things back somewhat, "only" being a crossover between The Flash, Arrow, and Supergirl, though in a surprising twist the 1990 Flash ended up making an appearance.note Legends of Tomorrow was left out for simplicity's sake since Crisis on Earth-X was a nightmare to coordinate around all four shows' schedule. That said, it is notably a Breather Episode compared to the previous year's crossover and is used to promote the then-upcomingBatwoman. Ultimately, it is just a setup for the following year's event.
The 2019-2020 event is Crisis on Infinite Earths, having been foreshadowed steadily since The Flash Season 1 and sporadically throughout the other shows, it featured five episodes that span all recurrent Arrowverse series, including Supergirl, Batwoman, The Flash, Arrow, and Legends of Tomorrow (the last of these actually began its forthcoming season after the Crisis). This was the last time Arrow got to participate, as the show ended in 2020 and its final season focuses on TonightThe Hero Dies in the lead-up to and the aftermath of the Crisis. It is an adaptation of the classic comic of the same name AKA the Trope Namer. In addition to the return of 1990 Flash, it also features a cavalcade of guest and cameo appearances from other DC live-action adaptations from outside of the Arrowverse continuity, including Tim Burton's Batman, Titans, the 1960s Batman series, Smallville, a merger of Superman: The Movie and Superman Returnsnote The Superman as portrayed by Christopher Reeve and Brandon Routh, Birds of Prey, Lucifer, Black Lightning, Swamp Thing, Doom Patrol and the DC Extended Universe. It also marks the debut of the titular character of Stargirl, ahead of the series' premiere on the DC Universe streaming service (which includes the aforementioned Titans, Doom Patrol and Swamp Thing)note A different version of Stargirl previously appeared in Season 2 of Legends of Tomorrow, played by Sarah Grey. As with the comic book, this crossover has a huge impact on the franchise, as it ends up merging the Black Lightning Earth with Earth-38 (the universe where Supergirl is set) and Earth-1 (where the other four shows are set) into a single new reality.
The Defenders (2017) is the self-contained type. It is an eight episode miniseries that sees Matt Murdock, Jessica Jones, Luke Cage, and Danny Rand team up to fight the Hand, a villainous organization built up in Daredevil season 2 and Iron Fist season 1. The crossover has lasting repercussions for the next phase of the Netflix shows, as Matt's "death" in the climax, and subsequent turning up at a convent, sets up the third season of Daredevil to do a loose adaptation of the renowned Born Again storyline.
Pre-revival, the show would occasionally have multi-Doctor special episodes. They had "The Three Doctors" (First/Second/Third), "The Five Doctors" (First/Second/Third/Fourth/Fifth), and "The Two Doctors" (Second/Sixth).
The two-part Series 4 finale, "The Stolen Earth"/"Journey's End", crosses over with spinoffs Torchwood and The Sarah Jane Adventures, and also brings in almost all of the Doctor's companions (a majority of the companions of the Russell T Davies era as a whole) to appear in the revived series to date. (Although it should be noted that this did not have any on-screen content or long-term consequences in either of the spin-off shows.)
"The Day of the Doctor" sees the Tenth, Eleventh, and retroactively introduced War Doctors work together to end the Time War. Eventually, all 13 Doctors (including the 12th, yet to debut Doctor) come together to end the Time War and prevent the destruction of the Time Lords.
"Twice Upon a Time" has the Twelfth Doctor team up with his original incarnation, the First Doctor himself.
A rather famous "Hurricane Saturday" event that happened on The Golden Girls and its spinoffs Empty Nest and Nurses (Nurses was technically a spinoff of Empty Nest, but go with it.) A hurricane hit during The Golden Girls and Empty Nest and the hospital of Nurses dealt with the aftermath. A similar even happened with a full moon, but that better fit as a Cross Through.
Ten years of Kamen Rider's Heisei era (and much later on, the franchise's 38 years up to that point) were celebrated in Kamen Rider Decade, where Decade (obviously the tenth) must travel across the Kamen Rider multiverse to save it from total destruction.
The Grand Finale of the Heisei era is Kamen Rider Zi-O, guest starring returning characters from previous series. Zi-O must avoid his fate of becoming an Evil Overlord in the future while preventing the antagonists from successfully grooming a substitute with the Another Riders they create, and the key to both parties' goals lie in the legendary Riders of the past.
The three crossover episodes that NCIS: New Orleans has shared with its parent show—one of which was its Back Door Pilot—had a case beginning in Washington on NCIS before evidence took them to New Orleans.
Power Rangers has done something like this a few times (not counting traditional two-season teamups), in what was originally meant to be the Grand Finale and as part of Milestone Celebrations.
The first, Countdown to Destruction during the sixth season, featured Dark Specter and the United Alliance of Evil declaring war on the entire universe. While the Space Rangers are the main protagonists, making this a Downplayed Trope, the Aquitar Rangers and Zeo Gold are shown fighting elsewhere.
Ten Red Rangers would later unite for a special mission in the tenth season, to stop the Machine Empire's remaining generals from re-activating Zedd's war Zord Serpentera as part of the 10th anniversary celebration.
The twenty-first season took the previous Rangers examples up to eleven with its Grand FinaleThe Legendary Battle, where every Ranger team to ever lead a season (including two from other planets, one from the future and one from an Alternate Continuity for some reason) come together to fight an Armada dwarfing the United Alliance of Evil declaring war on Earth. Most of the war is actually off-screen, with the Legendary Rangers fighting and rescuing people elsewhere while the season's own Mega Rangers fight the Emperor and high command; they all come together in time for the final battle against the last remnants of the army. This served as the 20th anniversary celebration, though the post-Channel Hop scheduling of the show delayed it to the 21st year.
Space Squad: Space Sheriff Gavan Vs. Tokusou Sentai Dekaranger involves Geki/the new Gavan and the Dekarangers teaming up to fight an alliance between Metal Heroes villains, with Madgallant as the movie's main villain. On top of this, the establishment of a space squad was done in response to another universe being dominated by evil, to prevent such an event from happening again.
Kamen Rider X Super Sentai Superhero Taisen is starting to look like this as well, with Decade's Dai-Shocker being ressurected, with their opposite number being the similarly-structured Dai-Zangyack Fleet led by GokaiRed of the Gokaigers. Both the Super Sentai 199 and the All Riders are at the very middle, wondering just what is going on.
The Super Hero Taisen series has become an annual tradition, though the focus has been shifting more and more toward the Rider side of things.
