Gravity Falls is known for this. Dreamscaperers ended with Gideon succesfully getting the deed, and starts destroying the shack. In season two, Not What He Seems ended with Ford stepping out of the portal and a lot of questions left unresolved. It would last to July when these questions finally got answered.
Taken up to eleven in the episode "200" which ends with three separate cliffhangers.
The episode "Professor Chaos", which appears to end on a cliffhanger: "Will Professor Chaos' latest plot succeed and be the final undoing of Earth? And which boy has been chosen as the replacement for Kenny? And which of these South Park residents was killed and will never be seen again?" (The first two were already the focuses of the plot, but the last ones comes out of nowhere). "These questions will be answered... right now: No, Tweek, and Ms. Choksondik."
South Park also squeezed the concept for all the humor and frustration they could in the "Who is Eric Cartman's Father" two-parter, "Cartman's Mom Is a Dirty Slut" and "Cartman's Mom is Still a Dirty Slut". They teased the fans mercilessly by splitting the two episodes up with an unrelated full-length April Fools' DayTerrance and Phillip episode.
Super Sunday: Each of the four animated shorts — Jem, Robotix, Inhumanoids and Bigfoot and the Muscle Machines — featured a cliffhanger-type ending each week for approximately 13 weeks, before each short came to a conclusion. These were as follows:
Jem: On the night of the Battle of the Bands (a talent contest with a finale similar to today's American Idol), series antagonist Eric Raymond — the corrupt manager of the Holograms' rivals the Misfits — holds Jerrica's friend Ashley hostage, hoping to extort money from the Holograms and force them to forfeit the contest to the Misfits (and with it, control of Starlight Music). It initially looks bad as the Misfits fall into a trap set by Raymond and his henchmen, but Rio arrives to save the day. In the end, as the Holograms are performing their ultimately victorious song, Raymond tries to escape with a briefcase containing the deed and thousands of dollars, but is ultimately arrested. Originally aired as the final installment of a 15-part cliffhanger-type series, this became the pilot episode for the daily cartoon.
Robotix: After 14 installments where the peaceful Protectons and warmongering Terrakors battle on the ailen world of Skalorr, the final battle commences where Terrakors' leader Nemesis steers the ship both factions are battling on into an asteroid belt. Eventually, the Protectons get the upper hand and are able to escape the ship shortly before it (along with the Terrakors) crash into an asteroid, presumably destroying the enemy forever. As the Protectons and their new human friends begin plans to rebuild their fallen world, Nemesis still lives. This was possibly the set-up for a daily cartoon that never came to pass. One aspect that is not resolved: the fate of Kanawk, an engineer who had betrayed his human friends to join forces with the Terrakors; during the final battle, he tries to sabotage lead Protecton Argus, but he grabs Kanawk and flings him aside against a wall, presumably killing him.
Inhumanoids: Following a climactic battle, D'Compose is re-sealed in amber casing and Tendril is imprisoned by the Granites. Finally, Magnokor succeeds in neutralizing Metlar even as Blackthorne is arrested by Earth Corps. Senator Masterson provides the Earth Corps team with a new headquarters facility, but a tissue sample secured from Tendril during their first encounter with the monster has mysteriously gone missing, which would be addressed when the series was picked up for a full 13-week season in the fall of 1986.
Bigfoot and the Muscle Machines: During the final installment, where the series protagonists — researcher Jennifer McGraw and Yank Justice, the owner of a monster truck show — battle the ruthless billionaire Adrian Ravenscroft at the Fountain of Youth, Ravenscroft drinks the Fountain's water and becomes a young man. But Yank and his allies ram the Fountain with their trucks, reversing the effects of the Fountain. Ravenscroft tries to flee but is turned back into an elderly man and walks into an alligator-infested swamp, presumably dying. Meanwhile, a large earthquake reveals a large treasure in gold, jewels and other rare artifacts from under the former Fountain. While several of the Bigfoot show members celebrate its discovery, Yank and Jennifer are unimpressed and drive off into the sunset. This is the only series that did not have a hanging conclusion that would lead into a possible (or actual) series.
Teenage Mutant Ninja Turtles (2003) its fond of the season-ending cliffhanger: three of its seven (As of 2008) seasons (1,2,4) end this way. Season 4's cliffhanger is notable for being in danger of never being properly resolved, since for never-quite-adequately-explained reasons, the people behind the show decided to skip season 5 and go from season 4 to season 6 without explanation, ignoring the cliffhanger. After several delays, the "Lost Season", as it came to be called, began airing on February 2008—a year and a half later than it would have, had it aired normally.
Also spoofed by the show on a number of occasions. At the end of one Holloween episode, the last few seconds reveal a surprise ending where Mr. Burns' head has been grafted unto Homer's body. The episode ends with a (fake) cliffhanger and a teaser for next week's episode, where Homer is denied a free spaghetti dinner because Mr. Burns has plans to meet with the queen of Holland that night. Of course, it's all a spoof and the next real episode has everything back to normal.
Beast Wars tended to pull out the stops at the end of a season. According to the writers, they were never sure if there was going to be another season, so they wanted each cliffhanger to possibly be the end of everything.
Subverted beautifully in Frisky Dingo. An episode ends with an almost literal cliffhanger as a woman is hanging from a ledge of a tall structure. Then, in the On the Next sequence, Killface says "Oh my God, she fell."
Season 1's first episode ends with the return of Nightmare Moon. The only forces capable of putting a stop to her plan are either inactive or nowhere to be seen. Resolved in part 2 when the Elements of Harmony are reactivated and used to save the day.
Season 2's first episode ends with Discord successfully mind breaking the mane cast (who cannot activate their trump card in this state) and establishing a reign of chaos upon Equestria. Resolved in part 2 when Twilight gets the gang back together, allowing them to put Discord back where he belongs.
Word of God says the season 3 finale is actually going to be continued in season 4, making it a 3-parter.
The first season (Mumfie's Quest) of The Magic Adventures Of Mumfie had its first twelve episodes end with the phrase "But what happened then will have to wait until next time!". This was removed from the film version.
Every episode of The Adventures of Rocky and Bullwinkle ends on a cliffhanger except for the ones which end a serial.
Super Robot Monkey Team Hyperforce Go! ended on a huge cliffhanger at the beginning of what would have been the main climax of the series. One more season of the show was intended, but Disney decided not to go for it, so everybody was left at the beginning of the final battle. As you might imagine, the fans were not amused.
Duckman ended with three characters — Duckman included — getting hitched, and his late wife Beatrice suddenly appearing alive and well at the end. Writer Michael Markowitz seems unwilling to divulge the ending, leaving it up to the fans to guess what may have happened.
Get Ed ends with Ol' Skool trapped in The Machine with Bedlam, and "sent away" by Ed. The series was not picked up for a second season.
The second season of Sonic SatAM ended with Snively emerging from the elevator he hid in, proclaiming that, now with Robotnik seemingly out of the picture, he would soon wreak havoc on the Freedom Fighters, accompanied by his new partner who we only see here as a pair of glowing red eyes (Word of God later stated those eyes to belong to Ixis Naugus). Next September, though, the third season did not show up.
Season 4 of ReBoot ended with the main characters trapped in the Principal Office, which had been taken over by the returned, upgraded Megabyte, who infected Phong and Welman and captured Enzo. At first it seemed that there wouldn't ever be a resolution (not helped by the death of Tony Jay, the voice of Megabyte). Luckily, an official online webcomic was released picking up where the series left off.
The very last shot of another Mainframe production, War Planets: Shadow Raiders, had the Beast looming over the helpless Planet Reptizar as it began to devour it. Especially frustating since they were building up for an epic showdown that failed to materialize. But looking at ReBoot, it seemed to be a signature "trademark" of the former Canadian-based animation studio to always keep ending every new season on a perilous thread, rather than simply giving audiences a self-contained story. Lessons about not being renewed for another season... were clearly not learned.
The first episode of Pepper Ann ended with "To be continued...". Strangely, there was no second part to that episode.
The children's show Between the Lions has a segment called Cliff Hanger, which stars the cartoon protagonist of that name. At the beginning of each segment, the announcer always says "We find Cliff Hanger where we found him last... hanging from a cliff!" And of course, by the end of each segment he's always right back where he started.
Cant...hold...on...much...LONGER!!! (And that's why he's called Cliff Hanger.)
Subverted in one episode whent he author decided to finally get him off the cliff because he's tried of writing the books. This angers Lionel, and he asks that the author get him back on the cliff. Which he does.
In Danny Phantom's season 3, Valerie found out that her employer is half-ghost and that Dani is half-human, but after that episode, nothing was ever heard of this ever again. In fact, Valerie practically fell off the face of the earth for the much of that season.
Undergrads ends with the love triangle between the main character and his two closest female friends still unresolved.
Spider-Man: The Animated Series: The series got Turned Up to Eleven for the last season with Mary Jane getting kidnapped by Hydro-man, Spider-Man gives chase, and gets his ass handed to him, but that's OK because MJ has Water powers now! But it turns out she's actually just a clone, then Madam Web comes in and says that Spider-Man must take part in a 3 episode Story Arc based on the 80's limited series, Secret Wars, after that he teams up with alternate versions of himself from other dimensions, to stop an evil version of himself that bonded with the Symbiote from destroying reality and gets transported to our dimension where he meets up with Stan Lee after that Madam Web vows to help him find the real Mary Jane... and that's it, that was the end of the last episode.
From That Other Wiki John Semper mentioned in an interview if he had continued on with the show, Spider Man would have gone through past time periods and found Mary Jane in Victorian England. Spider-Man would battle with the real Carnage portrayed as Jack the Ripper.
Zig-zagged: a handful of Mighty Mouse cartoons from the late 40s and early 50s with Pearl Pureheart and Oil Can Harry open with the resumption of a cliffhanger, when a previous episode never did exist.
The Fairly OddParents's Season 1 episode "Spaced Out!" ends with Cosmo losing his nickel. This ends with the screen zooming out with a narrator onscreen saying what happens next. Cue Yugopotamian aliens watching.
The series finale for Stroker and Hoop ended with Stroker and Hoop (Plus their friend "Double Wide") being dropped from a magnetic crane and over a giant cliff. Guess we're supposed to assume that they died.
A source states that Stroker and Hoop are killed in the fall and go to Hell. Coroner Rick and Double Wide have to die and get them out of hell. Supposedly, they do.
Young Justice was unfortunately cancelled, and to infuriate fans more than anything else, the finale ended on a massive cliffhanger. The cliffhanger? Darkseid.
Hey Arnold! ended on arguably the most controversial cliffhanger in western animation, given that 15,000 fans just signed a petition to resolve it.
The stinger for the final episode of Wander over Yonder shows a defeated Lord Dominator storming off through space and grumbling about how Wander and his friends will get what's coming to them. The camera then pans down to a crashed NASA probe which becomes enveloped in green lightning and from inside come monkey noises that give way to laughter like Lord Hater's. Unless the fans are successful in convincing Disney to greenlight a third season, we may never know what that was all about.