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.
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.
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.)
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?!" WELL GEE KIDS, I DUNNO!
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 Rot 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.
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 cloud.
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 the EPA agent near the end of the movie with the "treasure of I'm a weiner" joke. This spoiled the joke of "To be continued" followed by "right now" as the latter takes place before the former in the movie, showing there is more.
In 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.
Okay, Disney Channel may be the all-time king of this trope. 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.
Another one, the Phineas and Ferb 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!" ...Yeah, you get the idea.
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.
Not a trailer, but the original lobby card for the classic Daffy Duck short Duck Amuck flat out reveals who the mysterious animator is. It shows Bugs Bunny's hand with a paintbrush tormenting Daffy.
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.
A trailer for Total Drama All Stars shows Alejandro out of the robot.
Star Wars: The Clone Wars has restrained themselves 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!
For a particularly egregious one, the trailer for "Another Five More Short Graybles" (no, that's not a typo) spoils the riddle that is linked to all of the Graybles—they are all related to dads. It doesn't help the promos aired several times in the weeks leading up to Father's Day, and the episode itself premiered on Father's Day 2013 (June 17. 2013).
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.
Nickelodeon Australia once had a commercial for back-to-back episodes of The Adventures of Jimmy Neutron: Boy Genius that played this for comedy, with a narrator trying to summarize every episode in a single breath so the viewers wouldn't have to wait to watch all of them. He outlines about three episodes before becoming exhausted, and instead suggests they just tune in to watch back-to-back episodes.
This trailer for VeggieTales: God Wants Me to Forgive Them?!?!. If only they didn't include the clip of Junior forgiving The Grapes...
And the trailer following it for Dave and the Giant Pickle also spoiled that Dave knocked out Goliath with a peanut.
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.
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 occurs the very end of the episode.
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.
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.