The Sabrina the Teenage Witch episode "Inna-Gadda-Sabrina" extended its story through the following three shows that night: Boy Meets World, You Wish, and Teen Angel. Each show was set in a different time period in going with the theme.
It wasn't the first time - in fact, quite a few crossovers spanned the entire TGIF lineup from time to time, and even an occasional though less involving tie-in for all ABC weekly sitcoms during this period.
There was also one that was linked by Steve Urkel. He ends one episode of Family Matters by blasting through the Winslows' roof in a jet pack, and he crashes into the Lambert's roof at the beginning of the an episode of Step by Step where the plot centers around him. I forget if any other shows were connected as well.
The "Night of Elizabeth Taylor", broadcast on CBS around 1995-96, saw a diamond necklace lost by Elizabeth Taylor became a common plot element linking four SitComs — The Nanny, Cant Hurry Love, Murphy Brown and High Society — in one massive crossover. It was intended as an embedded advertisement for Taylor's new perfume, Black Pearls.
Disney did a triple-episode MMC with three of its shows. The show was entitled That's SoSuite Life ofHannah Montana, with one classed as a Suite Life episode, one as a Raven episode, and one as a Hannah Montana episode, where Hannah and Raven visited the hotel the twins live in.
Likewise, there's the Wish Gone Amiss triple-episode, except it is more loosely tied together. It all involves the title characters from Cory in the House, The Suite Life of Zack & Cody, and Hannah Montana making a wish on apparently the same shooting star. Each episode has its own method of returning to the Status Quo — Cory gets a literal Reset Button, Zack and Cody's wish was all just Zack's dream, and Hannah returns her life to normal when Jackson unwittingly wishes that the world did not know Hannah's double life.
There's also the WizardsOn Deck with Hannah Montana that goes by the same formula that the That's So Suite Life of Hannah Montana did, except with the stars visiting the hotel's ship instead of the hotel itself.
The Earth Day Special, which aired on ABC in 1990, was a huge crossover featuring just about every pop culture icon from The '80s in a very bizarre, thoroughly nonsensical plot.
Its conclusion even featured a brief cameo of Jim Henson As Himself.
Space Squad: Space Sheriff Gavan Vs. Tokusou Sentai Dekaranger and its prequel, Girls in Trouble: Space Squad Episode Zero are a variant on this. While the casts from bothseries are featured prominently, their adversaries comefromvarious other Metal Heroes series.
The direct-to-video special Kids for Character had Barney the Dinosaur and Tom Selleck appearing in-between clips from various childrens' cartoons and TV shows of the time as they teach the "six pillars of character" using the clips, ending in a musical number which featured The Flintstones and Thomas & Friends, among others.
Sprout's Preschool Musical On A Stick had Nina and Star narrating a Christmas story featuring characters from Sprout's original series and programming blocks at the time. During the finale number, "Nothing Better," the duo turns into stick puppets themselves and tell the viewers how they can have their own stick puppet musical.
Series:
In the CSI-verse, the original series crossed over with the Miami series in 2002 to serve as the pilot for the latter; the Miami series then crossed over with the New York series in 2004 to serve as that series' pilot, and the series crossed over again in a two-hour storyline in 2005. The original series also crossed over with Without a Trace in 2007 a two-hour storyline across both series. Following the departure of William Petersen (who opposed the spinoffs and did not appear in any scenes featuring the Miami team in "Cross Jurisdictions"), CBS put together a massive crossover in 2009 spanning all three series that involved the Las Vegas series' Raymond Langston going to Miami and New York while investigating a human trafficking organisation.
Dickensian is a mash-up series which features a wide range of characters created by Charles Dickens, before or after their main story happens.
The climax of the Second Doctor serial "The Mind Robber". D'Artagnan vs. Cyrano de Bergerac! Blackbeard vs. Sir Lancelot! Plus Gulliver and Rapunzel on the sidelines.
A very simple one, but the "iStart A Fan War" episode of iCarly included characters from Drake & Josh and Zoey 101, helping to fill out the Nick Verse.
Kamen Rider Decade is this in regards to the Heisei era Kamen Rider shows. The main character dimension jumps into alternate universes based on the 9 previous Kamen Rider series of the last 10 years (as well as the canonical universe of Samurai Sentai Shinkenger) and then a few other alternate universes based on Showa era series and some originals. Plus, the first movie features every main Rider created before Decade, even ones that only had appeared in one-shot movies previously.
The second movie, a Grand Finale, even includes some postmodern commentary on the "interesting effects on the fiction chosen" mentioned in the opening paragraphs. To quote the original universe Wataru: "The tales of the Riders were something that would be eventually lost to time. But because of Decade's battles, they will remain fresh in people's minds..."
Kamen Rider Zi-O takes Kamen Rider Decade's concept and doubles down by now being the 20th anniversary and the final chapter for the Heisei era. And this time, the main character time travels to meet his predecessors (or their associates if cameos couldn't be negotiated) and a few potential successors from possible future timelines. The movie Kamen Rider Zi-O: Over Quartzer even brings in Kamen Riders from non-canon entries such as comedian Noritake Kinashi's parody character Kamen Norider, Kamen Rider G, Kamen Rider Amazons Alpha and Omega, and a two-dimensional Kamen Rider Kuuga.
Law & Order crossed over with Homicide: Life on the Street a few times, until they eventually just decided they were set in the same continuity altogether, to the point of having John Munch, who originated in the latter show, permanently set up shop in the former.
The second western MMC took place a year after the first on what seemed to be an ordinary episode of Maverick called "Hadley's Hunters"; during the course of that hour he ran into people from 5 other shows: Lawman, Cheyenne, Bronco, Sugarfoot, and he stops by the office from Colt 45 but nobody was home (a reference to the show being recently canceled).
Strangely he also ran into the parking lot attendant from 77SunsetStrip which was set in the 1960s — I guess he had an ancestor who lived in the old west.
Once Upon a Time began as a fairy-tale mash-up (with specific references to the Disney Animated Canon) but developed into "any fictional work that is out of copyright or that Disney has the adaptation rights to". King Midas, Captain Hook, Lancelot (apparently Moorish in this timeline), Dr. Frankenstein, Peter Pan, and Mulan have all shown up. Snow White is the schoolteacher, Granny and Red Riding Hood run the diner, and the Wicked Stepmother getting therapy from Jiminy Cricket. Yeah. The show's run by Disney, why do you ask?
Similarly to Kamen Rider Decade, Kaizoku Sentai Gokaiger is a crossover series. But compared to Decade, in which the main character could only be Riders of the last decade, the Gokaigers can transform into any of the Sentai from the past 34 teams (barring Sixth Rangers - that is, until their own Sixth Ranger joined up). Additionally, Gokaiger establishes that all Super Sentai took place in a single universe.
states that dozens if not hundreds of television series have all happened within the mind of a young autistic boy living in Boston.