Adventure Time was a rather conspicuous case, as quite a few episodes (several of them highly-anticipated) were leaked before going to air:
"Fionna and Cake" and "Hot to the Touch" were leaked illegally on YouTube days before their premiere. Fans in denial about the former's ending revealing that the genderbent characters were the product of Ice King's fanfiction actually believed it was a dupe intentionally leaked by the show staff to throw fans off of a massive plot twist (alas, what they saw was what they got come Labor Day 2011).
Some members of 4chan managed to leak not only some clips from "The Lich" before its premiere, but also the first two episodes of Season 5, "Finn the Human" and "Jake the Dog".
Some episodes from Season 5, such as "Up A Tree", "Bad Little Boy", "Simon and Marcy", "A Glitch is a Glitch" and "The Party's Over, Isla de Señorita" were all leaked before their respective airdates with the latter episode being uploaded by accident on iTunes.
On December 2013, Cartoon Network listed eight episodes from the sixth season on their website, which not only spoiled the existence of Finn's father, but also revealed that Finn was going to lose his arm (though hardly anyone was surprised given how much foreshadowing it received).
Weeks before "Breezy" premiered, an annonymous user leaked several animation cells for "Something Big" and "Furniture and Meat" on 4chan, which spoiled Root Beer Guy's death and Finn recovering his arm. Said user also leaked some episode titles, such as "Evergreen" and "Jake the Brick."
On May 2015, Cartoon Network uploaded "Water Park Prank" (one of the episodes with a guest animator) on their website by accident, several weeks before its premiere.
Thanks to the security breach mentioned above, the majority of the episodes from Season 7, including the "Stakes" miniseries, were all leaked to the public.
Not an episode leak, but a leak nonetheless: on November 2017, a former member of the AT production crew posted a photo of a shirt on social media. Said shirt contained an official artwork related to the Series Finale, showing Finn, Jake, Fern, BMO, LSP, Princess Bubblegum, Marceline, Normal Man, Ice King, Betty and the silhouette of GOLB.
The season 2 finale "True Colors" was meant to air on May 1, 2021, but was delayed due to concerns surrounding Marcy being stabbed at the end. Unfortunately, since the scheduling change was announced less than 18 hours before its intended airing, iTunes didn't get the memo and they put the episode up for purchase on May 2nd. By the time the episode was removed, the damage was already done, and clips of the episode's final minutes were all over the internet by day's end.
Just like "True Colors", "All In" was accidentally released by Xfinity and screenshots and clips of the episode were surfacing on the internet. Fortunately, unlike the previous incident, fans made extra sure not to share any of the leaked footage on social media this time around. (The only pirated copy being available before broadcast being a cell phone recording probably helped.)
Avatar: The Last Airbender: The swordfight between Zuko and Jet from "City of Walls and Secrets" was revealed to the public early from an unauthorized online leak.
The Casagrandes episodes "Guilt Trip/Short Cuts" and "No Egrets/Meal Ticket" were put onto Amazon on December 9, 2020. Their official releases were January 29, 2021 and February 15, 2021.
Hazbin Hotel: About a week before the show premiered, the first two episodes got leaked online and were spread on several video sharing platforms such as YouTube and TikTok.
The series premiere was always meant to be viewable on Nickelodeon's website a few weeks before the TV premiere, with said early release predicated on the show's "Korra Nation" webpage getting 100,000 likes and shares on social media. However, but poor cyber-security (read: an administrative account with a default username and password) led to the first two episodes getting leaked into the wild before that goal was reached, prompting Nick to release the episodes earlier than planned.
Weeks before the season three premiere, four episodes towards the middle of the season (out of thirteen total) were accidentally uploaded to the website of Nickelodeon's Central American Affiliate, NickMundo. This lead to the TV premiere date being moved up, and the effect this had on the show's ratings is often blamed for the remainder of its run from the back half of season 3 onward to be released online-only.
The Loud House: The episodes "Future Tense", "Head Poet's Anxiety", "The Mad Scientist", and "Tripped!" were all leaked online on Amazon before their official airdate.
The Mr. Men Show: Many scenes from the second season were shown on certain animation demos on YouTube before the actual episodes they were apart of aired.
There were a few instances when episodes are posted to iTunes before hastily being taken down, but that usually results in the episode being downloaded and spread quickly. The first time this happened was "Hearts and Hooves Day".
A few days before the Season 1 episode "Winter Wrap Up" aired, songwriter Daniel Ingram excitedly shared a video of his song from the episode before it aired, but Hasbro quickly made him take it down.
An animator for Top Draw in the Philippines leaked Pinkie Pie's Smile Song (from "A Friend in Deed") a few months before the episode aired, but just the audio and in a poor-quality mono mix. Daniel Ingram wasn't happy.
The digital masters of My Little Pony: The Movie (2017) leaked just as it was about to hit theaters. Probably a good thing in this case, since somewhere along the line the visuals were messed with by adding a vignette.
During Season 7, several later episodes of the season were leaked online prior to their official air dates.
During the Season 7-8 hiatus, Hasbro suffered two large-scale leaks. Among the information found regarding My Little Pony were a handful of unfinished Season 8 episodes, titles and plot descriptions for every episode of that season, a new title sequence, a tentative timetable for the remainder of Generation 4, and very early concept art and ideas for Generation 5. Ultimately, all of the content of this leak proved to be genuine, with the only part that can't be officially verified being the concept art (for obvious reasons).
A Czech website selling children's bedding leaked the designs for this generation of the franchise a few days before it was officially announced.
Pages from a 2022 annual that showed a few plot points from the movie were also leaked on the Internet around the same time period.
On August 12, 2021, pages from the picture book adaption of the movie were leaked to the internet a few days before it was even released, revealing much of the movie's plot.
In late September, not too long before the release of the film on Netflix, there was an early-viewing event, during which the full film was recorded on a phone camera, which was then dissected for spoiler material and then distributed around the internet in a spoiler campaign.
"Adventure in the Elements" was leaked months in advance in Turkey.
"Really Small Problems", with Russian credits, was leaked hours before its intended airing.
The first two episodes of season two, "Separate Tides" and "Escaping Expulsion" were leaked onto YouTube following the early screening, a full week before the first one was supposed to air normally and two before the second was.
Some time before "Hunting Palismen" aired, an image leaked that showed the yet-to-be-revealed face of a certain character. Days after it aired, a Deleted Scene from that very episode was leaked online by a crew member.
An image from "Eclipse Lake" was also posted online before it aired.
The titles for "Follies at the Coven Day Parade", "Elsewhere and Elsewhen", and "Any Sport in a Storm" were leaked online in June 2021, many, many months before their actual airdates.
The titles for the three Season 3 episodes (including "Thanks to Them") and some of its background drawings were leaked online well before their actual airdates. Shortly before "Thanks to Them" aired, some screenshots from towards the end of the episode were leaked online, with many viewers unsure if they were real or not.
On January 5th, 2023, just over two weeks before it was meant to air, the entire duration of "For the Future" was accidentally made available on the Canadian iTunes store. By the time it was taken down, it had already been downloaded and shared widely.
SpongeBob SquarePants: A full animatic of "Gary & Spot" was uploaded by Vincent Waller, the show's co-executive producer, on his YouTube channel in late February 2019. It got pulled after just a few hours. "Gary & Spot" mistakenly aired in the United Kingdom at 4 AM on March 16th. It didn't officially premiere in the United States until July 27th.
The new opening sequence for the third season was shown at a convention, and despite being told specifically not to share it online, someone filmed it and shared it anyway. Shortly after, a better quality version was officially released online, likely in response to the leak.
During the two weeks of daily new episodes in November 2017, images from "Demoncism", "Sophomore Slump", and later "Night Life" and "Deep Dive" somehow found their way online, and contained quite a few spoilers for what would happen in each episode. A few weeks later, some images from "Stump Day" were also leaked, along with part of the episode's transcript, about a day or two before the episode was released.
It happened again with the episode "Booth Buddies," where images from the episode were leaked online, as well as part of the script.
Eden Espinosa, who voices Cassandra on Tangled: The Series, accidentally showed on her Instagram the title of a Season 3 episode called "Cassandra's Revenge", spoiling the big twist of the season 2 finale a good 8 months before it aired.
Teen Titans Go!: The episodes "Animals: It's Just A Word", "Top Of The Titans: Raddest Songs", "Butt Atoms", "Record Book" and "Bat Scouts" were leaked online weeks before they actually aired.
A promotional trailer showcasing the animation and the voices of some of the cast was leaked on the internet in January 2021. Although Mattel took the original down, it had already gone viral.
The cover of an upcoming character encyclopedia of the series was also leaked months prior to its release, and it showed James' and Kenji's reboot designs, thus confirming their involvement despite not appearing in the leaked trailer.
A few storyboards of a few episodes were also leaked, so did some toy merchandise.
In Italy, the first episode of the second part of the 2023 series was leaked by accident on Discovery+ before its release date of December 4th, but it has been removed.
A fan accidentally leaked storyboard images in the background of her photos taken during a tour of Studio Mir, then realized her mistake and asked everyone who saw her post to delete it. A different person refused to delete their copy of the images on Twitter in an attempt to blackmail DreamWorks into making the Keith/Lance ship canon (it didn't work).
Certain stills from the series finale were leaked three months before Season 8 had aired.
and screenshots and clips of the episode were surfacing on the internet. Fortunately, unlike the previous incident, fans made extra sure not to share any of the leaked footage on social media this time around. (The only pirated copy being available before broadcast being a cell phone recording probably helped.)