From Nickelodeon's Nicktoon, The Loud House, Even though the episode is called "No Spoilers, promos from this episode spoiled it anyway.
The episode "Squid Noir" From SpongeBob SquarePants was completely spoiled, when Nickelodeon launched a block called "You Bring The Color", and the website called youbringthecolor.com to use the pictures from the end of this episode so kids can color the characters from the ending.
Played with in Archer. The first episode of season 5 ends with a Fantasy Sequence of Archer's that serves as a trailer for the rest of the season. It did include some genuine spoilers, but they were interlaced with things that never happened (like Cherlene getting kidnapped) and things that happened in completely different circumstances (like Lana going into labor).
When Batman: The Animated Series originally aired on Fox, some of its promos could be pretty spoilerific. This one outright shows the cliffhanger ending of "Heart of Steel, Part 1" in it.
Promos for the first two Bionicle: The Journey to One episodes spoiled most of the ending by showing the sought-after Mask of Creation in the hands of the villains, from the very last scene. The only things not to get spoiled were Umarak kidnapping Ketar and mutating into a hulking beast upon putting on the mask, but these were already Spoiled by the Merchandise aynway.
Pick a trailer, any trailer, for any episode of a 2010s Cartoon Network series. You've basically seen the entire episode in under 30 seconds.
For a particularly egregious one, the beginning of the trailer for Teen Titans Go!'s "Justice League's Next Top Talent Idol Star" (no, that's not a typo) Shows that Robin, Raven, and Cyborg were the final contestants.
And if the trailer for "Giant Woman" is any indication, Steven Universe might be starting to fall victim to this too.
A promo shown during StevenBomb 4 showed a clip from "Barn Mates" spoiling the return of Lapis Lazuli, which aired a full four months later.
A less spoilery example, but one promo for the Regular Show episode "Go Viral" actually showed the ending of the episode (more specifically, the Farting Bear that was a Running Gag in it landing on Muscle Man).
The trailer for the series finale of Danny Phantom, "Phantom Planet", was almost completely spoiled just through the commercials. In one commercial alone, it was spoiled, though being outright stated by the narrator, that Danny loses his powers, gets them back again later, gets together with Sam, and enlists the help of every other ghost in the series to stop the Disasteroid.
During its original airing on The Disney Afternoon, the trailer for Part 1 of the DucktalesGrand Finale, "The Golden Goose", showed the cliffhanger ending of Huey, Dewey, and Louie getting turned into gold statues.
More Disney Channel screwing: In a Fish Hooks episode "Fail Fish", Milo has to pass a test in order to prevent himself from being held back a grade. This would have been taken as a surprise, if only they hadn't shown him holding the test in his hand while he shouts for joy, implying that he has passed it! You might not be able to catch it if you're not paying attention, but it's still there.
This print advertisement for The Flintstones special "Hollyrock-A-Bye-Baby" shows that Pebbles will have twins.
Black Phantom's defeat from the Hero Factory TV special was spoiled, shot for shot, on an on-line promo vid advertising the cartoon and characters (and their toys) on the LEGO website.
Nickelodeon showed various commercials of scenes leading up to the release of The Movie of Hey Arnold!. One of these completely spoiled that Helga finally confesses her love for Arnold. (They didn't show how he reacted to it, though.)
As if that wasn't enough, one of said ads actually showed a clip of Kim being shocked by Eric while he had an evil look on his face, which would pretty much give away that he's really working for Drakken.
The trailer for the season finale for The Legend of Korra was shown when there was still one episode to go before it, and spoiled that the Equalists will get control of the city and begin mass bending removal.
The YESvember music video on Nickelodeon's YouTube channel includes a scene from the The Loud House episode 11 Louds a Leapin', spoiling the reveal of Mr. and Mrs. Loud's faces.
A promo for the Motorcity episode "Like Father, Like Daughter" spoiled the ending in which Julie agrees to take over KaneCo should something happen to her father.
The trailers for the My Little Pony: Friendship Is Magic episode "Magical Mystery Cure" included a clip of Fluttershy saying "You look just like a princess!", completely spoiling the game-changing twist (That everyone already knew about) that occurs the very end of the episode. And then, Hasbro proceeded to plaster this plot twist all over all over their commercials and toys. Additionally, a sign depicting Twilight Sparkle with wings was actually sighted months before the episode was even announced, but was dismissed by most as an error from the artists.
To the great annoyance of viewers who had spent most of season 4 (if not longer) awaiting the frequently-referenced Equestria Games (and especially of Rainbow Dash fans), a promo for the final episodes of the season included a very brief clip of Spitfire crossing the relay finish line narrowly but noticeably ahead of Dash.
Also, in the Season 4 finale "Twilight's Kingdom", the Golden Oaks Library, where Twilight has stayed since moving to Ponyville is destroyed by the new villain.
In the Hub trailers for the finale as well, the Royal Sisters and Princess Cadence lose their Cutie Marks when they transfer their magic to Princess Twilight, leaving them helpless to being made hostages/prisoners of the new villain.  In the trailer as well, Twilight is seen taking off so fast, she leave a crater in the ground, against a blood red sky.
About the only thing the trailer for the finale didn't spoil was that said villain was a FIM version of classic villain Tirek.
In another instance of using The Reveal to sell more toys, Hasbro announced in 2013 a new set of "Rainbow Power" toys featuring the ponies covered in rainbows. Around the same time, the season 4 premiere introduced a mysterious locked chest whose contents were unknown. At the very end of the season 4 finale, the ponies finally manage to open the chest, and a magical rainbow comes out and makes them change into "Rainbow Power" versions of themselves as a Super Mode, allowing them to defeat the villain. Way to reveal your big end-of-season twist early in order to sell more toys, Hasbro.
In another minor example of the toys spoiling the theme: several months before the first advertisements for season 5 were out, a new set of toys were released, the theme this time being Cutie Mark Magic. Guess what the theme of season 5 is?
Phineas and Ferb has the episode "Candace Gets Busted". In order: It starts off as an intimate get-together, she has no problem inviting a few more people, but then it becomes a party, Linda calls the house and says they're coming home. Then guess what happens? "Young lady, you are so busted!"
To be fair, if you couldn't guess that was going to happen based on the title, well...
Then there's "Minor Monogram", where the commercials literally only showed the ending where Vanessa falls for Monty.
The theatrical trailer for Rugrats in Paris had Don LaFontaineproudly introduce Kimi as "the newest Rugrat", therefore cluing the viewers in on how the movie ends (Chuckie's dad marries her mom) and her incorporation into the show.
Parodied in The Simpsons with an old Radioactive Man film reel. A giant missile heads toward Earth, and cuts to a freeze frame while the announcer says, "Will Radioactive Man save the planet Earth? Find out next time!" The freeze frame shows Earth being split in half with a giant mushroom butt.
That's more a parody of old adventure serials. They routinely ended with something terrible happening (e.g., the hero's car skids off a cliff) and an admonishment to see the next installment to find out how the hero would avoid his terrible fate. Invariably, the next installment would be a total cheat (e.g., the hero jumps out of the car before it reaches the edge - even though he was clearly in the driver's seat as the car went over in the previous installment).
The promos for the episode "How Munched is That Birdie in the Window" spoiled the whole plot of Bart adopting a wounded pigeon and Santa's Little Helper eating it.
The trailers for The Simpsons Movie show the exchange among Homer, Bart, and Russ Cargill near the end of the movie with the "treasure of I'm a weiner" joke. This spoiled the "To Be Continued ... Right Now" joke as the latter takes place before the former in the movie, showing there is more.
The syndication promo for "When Flanders Failed" had Bart facing the bullies with his "karate skills". The announcer asked, "Will Bart get a black belt or a black eye?", followed by a cut to Bart hanging from a basketball hoop by his underwear.
Parodied in the South Park episode "Professor Chaos":
Narrator: Will Professor Chaos succeed? Which boy will replace Kenny? Which adult cast member will die? These questions will be answered... right now. No, Tweek, and Ms. Choksondik.
Though that may have been intended as a parody for the (in)famous "Who is Eric Cartman's real father?" 2-parter, since a lot of fans were ticked off when they got the Terrance & Phillip special after waiting for a month, instead of the continuing story.
The SpongeBob SquarePants made-for-TV movie Atlantis Squarepantis spoiled the climax where Plankton finds out he stole a tank that shoots ice cream. It wasn't like it was in a montage or anything either, they blatantly displayed it up front because they thought the punchline was funny enough to include. It got even worse when they put a Commercial Break Cliffhanger right before this moment.
Star Wars: The Clone Wars has restrained itself fairly well when it comes to foreshadowing Anakin's eventual transformation into Darth Vader, so naturally, Cartoon Network was eager to promote an episode where Anakin has a vision of what he will become. They show us some pretty sweet clips in the promos, but when the actual episode was aired, you realized they showed you the whole scene!
The season 2 midseason trailer showed somewhat more of the finale than Dave Filoni would have liked, in particular the Sith holocron, the return of Darth Maul and Ahsoka and Vader's duel. However, it did not spoil some particularly major plot points, namely Ahsoka's Uncertain Doom and Kanan getting blinded.
It's not a surprise to anyone keeping up with the show, but the season 4 trailer has a shot of Kallus in a Rebel uniform, which to anyone only familiar with his character from the first two seasons would be unexpected to say the least.
Steven Universe tends to get hit very, very hard with this trope. In the few days after Stevenbomb 4 alone, 2 clips were released on Cartoon Network's UK site as promos that reveal the return of Malachite and Lapis Lazuli. Especially infuriating due to the show's long hiatuses, which meant that the spoilers didn't even appear until almost 4 months later, when the episodes finally aired.
It's arguably gotten even worse with the "Steven's Summer Adventures" event, in which the network is releasing significant portions of pretty much every episode early as "sneak peeks." In a particularly irritating move, they have also uploaded clips and made news articles surrounding Bismuth being introduced, in which it is also shown that Amethyst has received another design change. It kind of makes you wonder whether any major development in the show will ever be allowed to go un-spoiled ever again.
The original trailer for Total Drama World Tour (back when it was still being called Total Drama the Musical) spoiled Leshawna's elimination; also, if you look quick, it also spoils Ezekiel's elimination as well, as you can see he doesn't get a bag of airline peanuts after DJ.
A midseason trailer for Total Drama World Tour spoils when Lindsay's elimination happens.
Trailers for All Stars not only shows Alejandro out of the robot, but also Lindsy and Jo's elimination.
Another World Tour example would be where a promotional flier for the season used to have accidentally given away the final two of the last season. This was especially bad because, at the time, the flier came out when there were about six contestants remaining. It was changed before the season aired, but the information had long been out by then.
An Italian trailer for Pahkitew Island shows Beardo's elimination.
From an episode preview on The Transformers, "But is this really the end of Optimus Prime? Tune in for tomorrow's exciting episode: "The Return of Optimus Prime".
However, the first time that aired, it didn't have the narration. Thankfully, newer DVD releases go without it (though of course the DVD's episode list does let you know that an episode called "The Return of Optimus Prime" is coming up.)
Not to mention all the trailers for the movie showing clips of Optimus Prime getting blown to bits while the Narrator asks, "Does Prime die?!"
Then the very next question the Narrator asks is: "Then, who will lead the Autobots?" There's not much need for a new leader unless the old one dies, ya know...
The 20th Anniversary DVD set prominently shows Hot Rod opening the Matrix on the inside cover art. This literally happens in the last scene of the film. And while all the other chapter titles are deliciously subtle, "Megatron Transformed" might have been playing its hand a bit too much.