By the sixth season, newcomers to Adventure Time were probably aware of the series taking place in a post-apocalyptic environment note While interviews before the show even premiered address this, and there are background hints even in the intro sequence, the show itself doesn't directly reference the issue until the end of the second season, that the Ice King is really a man that lived through the apocalypse and driven insane by the magic crown that protected him from the nuclear fallout, and a number of other things.
The titular Phantasm in Batman: Mask of the Phantasm is Bruce's ex-fiancée Andrea Beaumont. A clever viewer could figure this out anyway, but the toy division screwed up by releasing the Phantasm action figure, with removable hood, unmasked.
The CatDog episode "Vexed of Kin" spoils the events of "The Great Parent Mystery". Anyone who's seen the former before the latter would likely be confused by CatDog's adoptive parents being a frog and a Sasquatch-like monster.
The opening credits for the Chilly Willy short "Chilly Chums" credit Grace Stafford as "Woody's voice", sort of spoiling the gag about Woody Woodpecker making a suprise cameo.
In Code Lyoko, Aelita is a human girl. And Franz Hopper is her dad, and the creator of Lyoko. And later on William joins the group only to get permanently possessed by XANA. If you don't want to be spoiled, it is absolutely imperative you watch the series in season order.
The third and final season of Danny Phantom was aired in Latin American countries six months before they were supposed to be aired in the US. Impatient fans wasted no time snatching up the episodes, translating them, and posting them everywhere. Even if you didn't know via the internet that half of the ghosts had become more monster-like (Nocturne, Vortex, Undergrowth), Danny gets ice powers, and that Vlad becomes the mayor of Amity Park, said "surprises" were spoiled anyway through commercials and the episodes airing out of order.
In the first season, it was considered to be a bit of a shock when the Obviously Evil Asami turned out to actually be a Sheep in Sheep's Clothing all along, with her father being revealed as an Equalist. The fact that those aren't in spoiler brackets should tell you how much the community thinks of the twist as a spoiler these days.
The finale of Season 2 wherein Korra decides to leave the spirit portals open to allow humans and spirits to coexist peacefully isn't even spoiler tagged in the character sheet or the main page. Though considering the events of that season lead directly to a Mass Super-Empowering Event that rebirthed the air nation from nonbenders, including the Big Bad, setting the stage for the remainder of the show, this was inevitable.
The final scene of the series itself, wherein after a truck load of Ship Tease over the course of the last two seasons Korra and Asami become the true, and final, Official Couple of the series. This quickly become one of the most talked about and well-known elements in the show amongst fans, critics, and even those who have never seen the show before (to the point that it has arguably reached It Was His Sled levels).
In the later Looney Tunes short Duck Dodgers and the Return of the 24 1/2 th Century, Marvin the Martian asks Duck Dodgers to see Gossammer, who Dodgers assumes is a beautiful woman, but ends up being a big red monster from two previous shorts. This is the first time that character would be referred to by that name (previously being a nameless monster in Hair-Raising Hare and named Rudolph in Water, Water, Every Hare, but afterwards, Gossammer would come to be the characters official name.
The second season premiere, "The Collector", reveals that Gabriel Agreste is secretly Hawk Moth, and that his assistant Nathalie is his Secret Keeper and The Dragon. Both of these facts inform their characterization in later episodes.
Twilight Sparkle becomes an alicorn princess at the end of the third season. With the show going on to have six more seasons after this change, this no longer matters whatsoever. But at the time, not only was this featured in merchandise, but the Spin-Off movie for the show and its promotional material, My Little Pony: Equestria Girls, takes place just after her ascension and openly shows her pony form portrayed with wings. Fans did little to hide this plot development from new viewers, too, but there was also advance publicity in mainstream media outlets such as Entertainment Weekly. This is without delving into foreign territories, where Equestria Girls was released before season three (such as in Germany, Portugal, and Japan).
Spirit of Chaos, Discord, made a HeelFace Turn during Season 3. He has made enough casual appearances in later episodes, with two season finales showing him help save the day, that people new to the show might not realize he used to be a villain.
Due to her being the protagonist of the film (and all later entries within the spinoff franchise, for that matter), the promotional synopses, preview trailers, and teaser clips of Equestria Girls: Rainbow Rocks casually spoil that Sunset Shimmer, the Big Bad of Equestria Girls, pulled a HeelFace Turn at the end of that movie.
Midway through season 5, the Cutie Mark Crusaders finally receive their cutie marks, which are clearly visible in their subsequent appearances, as well as during the theme song from season 6 onward.
Watching anythings from the final four seasons reveals that Starlight Glimmer pulls a HeelFace Turn and becomes a main character. Meanwhile, the final three seasons spoil the HeelFace Turn of nearly the entire Changeling race.
In ReBoot, Megabyte and Hexadecimal being siblings came as a surprise in the late second season. Nowadays, it's common knowledge, is constantly referenced in subsequent episodes, and any biography of them will list this fact fairly early on.
In The Simpsons, anything related to Sideshow Bob (one of the most merchandised characters outside of Our Favorite Family) will point out that he wants to kill Bart, or at least that he is a recurring villain. In his premiere episode, however, Sideshow Bob being evil was actually The Reveal, coming off as Krusty the Klown's bumbling sidekick up to that point. Most of Sideshow Bob's return appearances mention this fact (along with other crimes he attempted).
The DVD box art for season six reveals that Maggie shot Mr. Burns, even though the commentators for the first part of the episode in question are tight-lipped in the hopes that we'll buy season seven.
The DVD boxset for season 14 of South Park reveals Mysterion's identity in the cover art.
Darth Maul still being alive is not much of a secret anymore after the Season 4 finale, and it was a big surprise back in Season 3's "Witches of the Mist". The advertisements for the Season 4 finale heavily promoted Maul's return, he's on the box art for the season's DVD/Blu-Ray case, and he was featured in the Season 5 trailers and the 100-episodes milestone video.
The trailers for the show's revival season reveals a lot of major spoilers from the past seasons; the SDCC '18 trailer gives away Bo-Katan's HeelFace Turn, and the Celebration Chicago trailer spoils Ahsoka leaving the Jedi Order and the deaths of Fives, Echo and Hevy.
Darth Maul is back, and he blinded Kanan. Any picture of him from Season 3 onward spoils this, even if it's just "Why is he wearing a mask?"
For the whole season, it's that AgentKallus had a HeelFace Turn, became a Reverse Mole (namely, Fulcrum II), and in the season finale was exposed and arrested before managing to escape and join the rebels. In particular, the first season 4 trailer has a brief shot of him in a rebel officer's uniform, which is going to be hard to explain to anyone who hasn't watched the show up to "The Honourable Ones" at least.
Chopper Base, established by Phoenix Squadron on the planet Atollon near the end of season 2, was attacked and destroyed by the Empire, resulting in the deaths of many of its members, including Commander Sato, who goes out in a Heroic Sacrifice killing recurring Imperial Admiral Konstantine via Ramming Always Works.
Season 4's midseason premiere, "Jedi Night", features the death of a major character: Kanan. Disney XD's YouTube Channel spoiled it in the title and thumbnail of a video the DAY the episode aired. When it had not yet aired in the U.K. and Canada among other places. NOT COOL. The thumbnail also spoiled Kanan shaving his beard.
Garnet being a fusion of Ruby and Sapphire was a big reveal in the season one finale, making it difficult to discuss her without bringing this up. This tends to be the first thing new viewers learn about her character now.
That Rose Quartz was really Pink Diamond was revealed halfways into the fifth season and quickly became one of the most discussed aspects of the show. This wasn't helped by Cartoon Network themselves releasing a 17-minute video a few weeks after the reveal that highlighted a decent amount of the foreshadowing that built up to the moment. The movie, including the characters section on the official website, also has no qualms about revealing this information, as it's openly discussed in the Storybook Opening and factors into the plot.
The promos stopped hiding Blindstrike's true identity after the Flex Fighters discovered it (in episode #12, "Endgame"), as well as Blindstrike and the Flex Fighters teaming up against Stretch Monster afterwards. Additionally, Blindstrike's figurenote which Wal-Mart's website describes with exorbitant detail about Blindstrike's double life has a removable faceplate, but the instructions don't show how to remove it, or how Blindstrike looks without it.
Any story from the Gamebooks special "The Breakout" onwards would spoil twists of the first season finale, "The Age of Flexarium" (such as the true identity of Stretch Monster, and Dr. C's reasons for scheming against Rook), and their changes to the status quo.
Tangled: The Series takes place right after the original movie and as such many character aspects and plot elements which were spoilers for the film are present as established facts in the series. For example: "Flynn Rider"'s real name being Eugene Fitzherbert, which is the name used for him in the series; Rapunzel losing her magical long hair at the end of the movie; the fact that Mother Gothel dies at the end of the movie; and, perhaps most blatantly, the fact that Rapunzel is, indeed, the Lost Princess of Corona.
The true nature of the Teenage Mutant Ninja Turtles (2003) version of Shredder became this, particularly after it became necessary to qualify him as such in order to distinguish him from the other versions of the character.
This is also immediately spoiled (and lampshaded) in Turtles Forever if one hasn't seen the 2003 series at all. The 80's Shredder's reaction to the 2003 Shredder's true nature is likely to mimic that of viewers unfamiliar with the 2003 series. Likewise, it probably worked vice-versa (fans of the 00s series would be surprised by the 80s version's incompetence).
The reveal that Irma was secretly a Kraang spy, Kraang Sub-Prime set the events of the Season 2 finale and first half of Season 3 in motion. Any newcomers who first see "her" in later episodes will likely be confused as to why she is being so friendly with Casey and April in earlier ones.
Fans who came in after the first season are going to have the bombshell dropped on them that Karai is actually Splinter's biological daughter Miwa spoiled almost immediately. Heaven help those who end up watching episodes from Season 3 or after before the first twoas they take place after Karai ends up becoming a mutant herself late in the second season.
Commercials for the first season DVD of Total Drama blatantly showed that the final three would be Owen, Gwen, and Heather, and a press release for Total Drama World Tour revealed the final two of Action when the season was still underway with six contestants left. They eventually caught this mistake and changed it to two other contestants, but the damage had been done.
World Tour itself has been plagued by this, first because Cartoon Network's commercials made it possible to figure the vote-off order, and then because Australia got the episodes and aired them in quick succession. Bizarrely, Canada (TD's home land) is getting the show after just about everybody else; even before Australia, most Canadian fans were YouTubing episodes after America aired them rather than waiting months for them to come out there.
Revenge of the Island was being aired in French long before it aired in English. The language barrier, of course, did nothing to hide the elimination order, apart from the alternate ending.
Pahkitew Island started airing in Italian before it was aired in English. Given that new episodes air in Italy every day, by the time the show premieres in the US, a winner is already declared.
The official Facebook page for Pahkitew Island has been posting who was eliminated a day after each episodes airs. Like the Italian version, a new episode aired every day.
The true identity of Longarm is one of the most shocking reveals of Transformers Animated... so naturally, it was all over the Internet in a pretty big hurry. Then the toy came out. At this point, it's probably not likely to surprise many people any more.
It was over the Internet before anyone who talked about it saw the episode. It aired in Dubai and this was about all the blurry screencaps could tell us.
Considering that he's a major player in the third season, if you're still not aware you're either blind or not up to date yet.
By at least halfway through the first Season of Transformers: Prime, you should be aware that Cliffjumper is dead. It happened in the first five minutes of the show and it frequently gets brought up. The initial announcement of its sequel series, Transformers: Robots in Disguise, does this to the events of the final episodes of Prime, including Bumblebee regaining his voice box (part of the announcement was Will Friedle reprising the role of Bumblebee), the restoration of Cybertron, and that Bumblebee taking over the reins of leadership following Optimus Prime's Heroic Sacrifice.
Season 3 of The Transformers, taking place after The Movie, spoils a lot of what happens in it, such as Unicron being reduced to a head orbiting Cybertron, Hot Rod becoming Rodimus Prime and leader of the Autobots, Optimus Prime kicking the bucket, a few other Transformers following Prime along for the ride and Starscream getting killed by Galvatron, who is an upgraded Megatron. While the movie came out in the US in August 1986 and season 3 debuted in American syndication in September of that year, other countries didn't get the movie until later, with Japan in particular not getting the movie until 1989, by which time they were on their third sequel series to the third season!
Looking at the promotional images from Trollhunters spoils that the Staff of Shadows Angor Rot wields eventually ends up in Claire's hands, as well as that Toby eventually acquires a warhammer at some point.
The Venture Bros. creators have a habit of giving large spoilers during episode commentary on the DVD. Played straight when one commentator points out that spoilers have been given and the collective response is "No one watches the commentary before they watch the actual show!". Later subverted during a convention when the shows creators are confronted by a fan whose friend saw the commentary before the episodes being spoiled.
In Winx Club, Bloom's status as the long lost princess of Domino is never kept a secret, despite it being a major plot twist in the first season. In addition, there is Sky and Brandon's identity switch. The actual prince Sky is Bloom's boyfriend, while Stella is actually dating Brandon, Sky's squire. All promotional material refers to them as their real names.
The latter is averted in the specials Nickelodeon made.
The plot of Young Justice's second season, Young Justice Invasion, is set in motion as result of the events of the finale. This also includes a character who is Walking Spoiler as they give a MASSIVE twist in the first season's finale.