Of course, some of this depends on your definition of "spoiler". Given that a trailer consists mostly of clips from the movie itself, a fair bit of spoilerage, in this case footage from a later part in the movie, is often inevitable.
There is also the matter of context. An action movie, for example, may preview a random fight scene between two characters, but when you actually see the movie itself and realize that the other dude the hero was fighting happened to be his best friend in the beginning (which may or may not have also been part of that trailer), you realize in retrospect that the trailer's fight scene was actually a foreshadowing or Chekhov's Gun about the betrayal that occurs later.
When a certain event is a foregone conclusion, like with The Film of the Book or films Based on a True Story, it's again debatable that it's really a spoiler. So if you read the examples below from a movie you haven't seen (either you have no intention of seeing it, or maybe you should reconsider continuing past this part), and find yourself thinking "I didn't even know that was a spoiler", don't worry about it.
It should also be noted that in many cases, commercials released after a film's first week or so will contain a lot more spoilers than those leading up to the premiere.
A related phenomenon can occur with DVD menu intro screens, which often include clips or montages of footage from the movie/episodes, thus potentially revealing them to the viewer out of context, before they have a chance to actually start playing the film proper. Often these will give away major plot points before the viewer has a chance to even start the film. These can be even more effective at spoiling the film's plot than trailers, since a viewer might plausibly be expected to go days between seeing a trailer and finally seeing the related movie, which might give them a chance to forget things from the trailer. With menu intro screens, on the other hand, the viewer is being shown clips from something that they are moments away from watching.
Also related are the trailers which run immediately previous to the show you have already sat down to watch. Known as a Precap and often starting with "Next, on X:", these segments spoil you on things you would just know in the next 30-60 minutes on a show you have already decided to watch. These are intended to pull in the new viewer, but can seem unfair to those already into a show as you are most likely to be already watching at the beginning of the episode.
In comic books, solicitations of future issues can spoil not only the current story line but future ones as well. It's easy for readers to deduce which characters that will live or die, new characters that will be introduced, and major events that will happen simply by looking at covers and descriptions for issues a couple month down the line. A common way publishers get around this is to have the solicitations be as vague as possible and have advertised covers be incomplete, edited to remove potential spoilers, or be a variant to the main cover. However, it's not uncommon for publishers to purposely leak major plot points of their big event comics in order to generate interest for the book.
Fanfic summaries can be especially bad about this. In fanfiction writing, it's conventional to warn the reader beforehand about any plot points they might potentially dislike such as specific romantic pairings, a major character death, or a tragic ending. This, however, can easily kill off the tension if these are meant to be a surprise. This is however justified by the intense Flame War the writer most probably wants to avoid, and to not have to deal with some vitriolic Ship-to-Ship Combat in the feedback. Also, many fanfic readers prefer to know what they're getting into so as to be able to steer clear of stories featuring a rival pairing.
This trope can lead to Trailer Joke Decay.
As this trope concerns spoilers, unmarked spoilers follow.
- The Clone Saga: Marvel announced several months in advance that the Spider-Clone would return with interviews in Wizard magazine and advertisements. During this time, the titles saw a "mysterious drifter" covered in shadows spying on Peter and visiting the graves of Parker's parents. It was obvious that it was the clone but the titles still treated it as some sort of huge mystery even to the point where, when they revealed the clone's face, it was played off as some surprise twist.
- Marvel tried to keep the plot of the infamous One More Day under wraps, but an interview with J.Michael Strazynski revealed exactly what would happen, who would be involved, and what the outcome would be. Despite this Marvel ran a promotional campaign for the story which gave readers a multitude of options as to who could help Spider-Man in his darkest hour, but thanks to the JMS interview, everyone knew it was Mephisto, they also knew the outcome resulted in the retconning of the Spider-Marraige in the 616 continuity.
- When Marvel announced that X-23 would be joining the cast of All-New X-Men, they effectively spoiled that she'd survive the events of the then-still running Avengers Arena series, ruining the supposed Anyone Can Die element. The writer of Arena even jokingly acknowledged this at New York Comic-Con.
- In 2011 Marvel Comics was especially bad about this, saying that now they'll probably kill off a major character every quarter to raise sales:
- February saw the death of The Human Torch in the conclusion of the "Three" arc in Fantastic Four, the title of which heavily implied one of the titular four dying. While the story's title managed to avert the typical Oh, and X Dies nature of a lot of "The Death of X" stories Marvel ruined any potential surprise by spoiling it to the press the day before the release.
- Early June saw the death of Bucky Barnes, the second Captain America, in Fear Itself #3 to the surprise of almost nobody as Marvel had already announced that original Captain America Steve Rogers would return to the uniform a month later (due to the movie). The only reason it didn't make any sort of media splash was because DC Comics one upped them the day before by announcing their reboot.
- Secret Wars (2015) was marketed on the premise that the Marvel Universe was being Killed Off for Real, and that nobody was sure what new universe would be taking it's place. The subsequent sneak peeks for books like All-New, All-Different Avengers and Invincible Iron Man, coupled with Word of God confirmation that books like Ms. Marvel (2014) would continue, more or less confirmed that the "new" Marvel Universe was essentially the same one as the old one.
- This was solidified when Marvel released its new line-up of post-Secret Wars books, which spoiled a number of plot points about the ending, such as the Marvel Universe being restored, and characters like Old Man Logan joining the mainstream continuity.
- In 2013, DC Comics spoiled the death of Damian Wayne, the most recent Robin, days before it actually occurred in Batman Incorporated. This of course gave media outlets ample time to cover the story.
- After initially teasing it as one of the big mysteries of the All-New Marvel NOW! launch, Marvel informed the New York Times that the new Ms. Marvel would be a Muslim teenager named Kamala Khan.
- In yet another Marvel example, a released script for Mighty Avengers spoiled that the new Ronin was Blade a full eight months before the writer's intended reveal date.
- A few weeks before Forever Evil kicked off, DC announced that Trinity War would see the Crime Syndicate of Earth 3 removing the Justice Leagues from the picture & taking over the Earth, with Lex Luthor's alliance of villains fighting back.
- Marvel basically confirmed that Hunger would end with a Failure Is the Only Option scenario by revealing that their next Crisis Crossover for the Ultimate line, Cataclysm, would see Galactus reaching Earth and beginning his assault. To those unaware, the entire premise of Hunger was that Silver Surfer and Rick Jones were desperately trying to stop Galactus before he could get to Earth.
- A plot point in Original Sin involved Nick Fury recruiting Black Panther, the Winter Soldier, Gamora, Doctor Strange, Emma Frost, Scott Lang, and The Punisher for a mysterious purpose. Eventually, it was revealed that Fury was dying of old age, and needed one of them to be his successor. Marvel treated the news as The Reveal and made it seem like a serious "Who will Fury choose?!" Cliffhanger, despite the fact that they'd already revealed that Fury's successor would be the Winter Soldier several weeks earlier.
- The same basic thing happened in Rick Remender's Captain America run. Marvel revealed on The Colbert Report that Sam Wilson would be the new Captain America, yet still treated it like a big secret in-story, and even tried to make it seem like Sam had died pulling a Heroic Sacrifice just before he emerged as the new Cap. This was then lampshaded when Sam was finally unveiled onscreen to his Avengers teammates. He immediately says that there is no drama in this reveal, and that everyone must have known by now that he was going to be the new Cap.
- The Reveal of DC Universe: Rebirth #1 is that the Pre-Flashpoint Wally West still exists and remembers the previous continuity. Naturally, DC revealed this in official previews and sanctioned reviews about a week before the issue hit stands. However, the reveal was leaked online a few days earlier, so it's unclear if DC would have released said previews and sanctioned those reviews if it weren't.
- The Vision established a pretty effective Anyone Can Die tone, since Vision's wife and kids are all new characters without any ties to the rest of the Marvel Universe. Marvel spoiled that his daughter, Viv would survive the book when she was confirmed to be part of the new Champions series.
- This was pretty much averted with Batman: Endgame, as none of the early covers were revealed and all the solicitations just said was that it was an important story and one would have to read the first issue to find out that the Joker was making his post-Death of the Family return.
- A full three days before the release of the final issue of the controversial Secret Empire mini-series, The New York Times spoiled that the final battle would see the return of the heroic, classic Captain America, who would do battle with his evil, HYDRA-aligned counterpart. While that was already a Foregone Conclusion for the most part, the paper even published the pages showing Good Cap defeating Evil Cap by using Thor's hammer.
- Heroes Reborn was advertised during Onslaught, revealing well before the Onslaught: Marvel Universe one-shot that concluded the story that Captain America, Iron Man, The Mighty Thor, the Bruce Banner personality, Giant-Man, The Wasp, Hawkeye, the Scarlet Witch, The Vision, The Falcon, the Black Panther, Crystal of The Inhumans, the Fantastic Four, and Doctor Doom would (seemingly) fall in battle.
- According to the premiere solicitation for Tom King's Strange Adventures, the fates of Adam Strange and the world would be determined by "a surprise DC hero". Nevermind the fact that the March 2020 solicitations for the Dollar Comics reprint line has a reprint of Swamp Thing #57 that's advertised as being "Offered to coincide with STRANGE ADVENTURES #1".
- The summary of Thirty Hs is basically a summary of the events of the first few chapters.
- Fan Fiction Dot Net put a massive TRON: Legacy spoiler in their "filter by character" view. This was before the film hit DVD.
- In the fic MVP, the accompanying image outright spoils its punchline (the identity of a character).
- In the Total Drama story, Legacy, the brief Introduction suggests pretty strongly that the story's "for want of a nail" premise will end badly for a certain character. Sure enough, it does.
- The TV commercials for 9 try their best to hide things by cutting rapidly from scene to scene, but they still manage to spoil 2's funeral, the destruction of the Fabrication Machine, the Cat Beast's death, and several of the dolls having their souls sucked out by the talisman.
- The trailers for the US release of The Adventures of Panda Warrior spoils the ending by showing Patrick fighting the villain. Though, to be honest given the quality of the film, they probably wanted you to know how it ends to prevent you from watching it.
- The trailer for An American Tail: Fievel Goes West is a special case. It didn't spoil itself so much, but the fact that the trailer of the sequel movie (which begins with the ending scene of the previous film, with Fievel riding a pigeon with his sister and asking if he could see more of America) was included at the beginning of every VHS release of the first movie after 1991 meant that it revealed the ending of An American Tail before the movie even started, spoiling that Fievel finds his family for anyone who was watching the movie for the first time.
- The trailer for Don Bluth's Anastasia spoils the fact that Anya is Anastasia, and then shows her in the palace, in formal attire, being addressed as "the princess" and "Your Highness". The previous spoiler isn't so bad, since it's to be expected, but the latter is a big spoiler.
- The trailer for Bartok the Magnificent spoils Ludmilla's transformation into a dragon.
- The trailers for Big Hero 6 never really tried to hide the fact that Tadashi dies, although it happens early in the film. UK advertising reveals that Hiro wants to kill Yokai.
- The toy commercials for the BIONICLE's Glatorian Legends line of figures showed glimpses of the then-upcoming movie, The Legend Reborn. Among them, the very shot of the four heroes unleashing the final blast at the Skrall and Bone Hunter army, from the end of the climax! Website promos also spoiled Metus' transformation into a snake, thereby revealing that he was to be the traitor. This was also spoiled by the site's behind-the-scenes videos, which showed the actors recording lines — fans have narrowed the identity of the traitor down to Berix and Metus, and then the clips showed Berix's VA talking to the traitor.
- Near the end of the second trailer for The Book of Life, it happens in a brief second but one can see the Sanchez ancestors near the climax.
- Not as bad as most cases, but there are a few scenes in the trailer for Captain Underpants: The First Epic Movie that show Captain Underpants with superpowers, proving that he does gain them at some point in the movie.
- Chicken Run: As shown in one of the TV commercials, the chickens manage to escape by making their own pedal-powered plane!
- Coraline. The trailer shows the scene where the Other Mother gives Coraline the buttons and sewing needles.
- Promotional material for The Emoji Movie barely tried to hide the fact Smiler was a villain. One trailer, for example, had her threatening the protagonists for leaving the phone.
- Finding Dory had a bad case of this:
- Later trailers and commercials for the film spoil the scene where Hank gets poked by a kid and inks in the touch pond, causing the kids in the area to run away. It got to the point where said clip was shown during shows where Ed O'Neil, Hank's voice actor, was interviewed such Entertainment Tonight.
- The clip used to introduce Hank in this April Fool's prank is part of the scene where Dory reaches her old home at the Open Ocean exhibit.
- This Subway commercial for kids' meals based off the film uses clips that spoil some major plotlines in the movie, including the fact that Marlin and Nemo wait for Dory in the tanks that are going on the truck to Cleveland.
- The film's Japanese trailer shows the otters bringing Dory into the truck bound for Cleveland as well as Destiny jumping out of her enclosure, both from the climax of the film.
- A fan-edited trailer parodies this. The video makes some Retraux attempts at emulating an ad from The '90s, with a love ballad playing in the background, some idents from Disney's VHS tapes serving as bookends, and a clip revealing some major points about the climax (the scene in which a freezing Anna tries to stop Hans from killing Elsa) appearing before the title card.
- Official trailers show some spoileriffic clips, such as Elsa bringing back summer and saving Olaf from melting. A more subtle example is Elsa giving Anna a hug, when Elsa had previously been too scared to approach, much less touch, Anna. It's also apparent on closer examination that this happens after Anna's aforementioned Heroic Sacrifice frees herself from the curse.
- Home release trailers for Frozen II reveal the costume change Elsa receives in the last fifteen minutes of the film.
- The Good Dinosaur took this Up to Eleven. Most of the trailers spoil Poppa's death, Arlo's friendship to Spot, the meeting of Forrest Woodbush, Arlo helping the cowboy T-Rexes, Arlo encounters the main villain, Thunderclap and also the climax. In short, the trailers just showed you the entire movie. This could be one of the reasons why the film didn't perform so well at the box office.
- The trailers for Hoodwinked Too! Hood vs. Evil stress the fact that Red and co. are trying to save two innocent kids from a wicked witch. Save for the one which shows said "innocent" children with creepy grins saying: "You've been hoodwinked! Too!"
- How to Train Your Dragon 2's trailer takes no issue with spoiling The Reveal of Hiccup's mother.
- Inside Out:
- The Japanese trailers run on this, so they aren't to be seen by anyone (besides people who don't mind about spoilers) before seeing the film itself.
- Some of the American trailers spoil the movie as well. Two notable examples were the clip where Joy rides in Bing Bong's rocket to the top and we can clearly see that he is not in it, and the scene where Joy and Sadness return to Headquarters.
- Kung Fu Panda:
- The DVD trailer for Kung Fu Panda 2 completely spoiled the fact that Po's biological father, and several other pandas, are still alive, which wasn't revealed until the last few seconds of the film.
- The DVD trailer for Kung Fu Panda 3 spoils literally almost the entire movie, including Kai's destruction of the Jade Palace, Po training of the panda village to fight against Kai, and the final battle in the Spirit Realm.
- The original trailer for Lady and the Tramp, all the way back in 1955, shows us that not only do Lady and Tramp get together, but have puppies to boot.
- One preview of The LEGO Movie showed bits from Emmet's vision, including a live-acted human hand. Also, Bad Cop's fate was revealed by his toy, released prior to the movie. The LEGO sets actually gave away many details, such as Bad Cop's parents getting frozen in glue or Uni-Kitty disguising herself as Business-Kitty, but not always the exact interpretation of the scenes.
- The trailer for Life's a Jungle: Africa's Most Wanted clearly shows the ending of the movie.
- The Lion King's trailer spoils most of the plot. Movie posters show Mufasa's ghost as well.
- Meet the Robinsons had a trailer that showed the future Lewis in a group with the Robinson family, with his arm around Franny from one of the final scenes in the movie, making the big reveal completely predictable.
- Megamind probably had one of the worst offending trailers, in which it revealed that Metro Man was not dead and had simply given up superheroing. The entire film revolved around the fact that Megamind was trying to cope with having killed Metro Man.
- This official preview for Monsters University spoils the entire plot.
- This Mulan trailer in which it shows the Emperor's big speech at the end, which in turn gives away big chunks of the movie. It even shows the scene with her turning around and all of China bowing to her.
- The original BMG Video trailer for Mumfie's Quest shows Mr. and Mrs. Admiral being reunited, Whale being freed from a net by the electric eels and part of the scene where Secretary of Night gets turned into stone by the Cloak of Dreams.
- In ParaNorman, the trailer spoils nearly every joke, even if it does not spoil the plot for the most part. However, the Japanese trailer did spoil the plot.
- Watch any of the commercials for The Powerpuff Girls Movie and you've watched the whole thing (and this could've been why the movie didn't do so well). It also doesn't help that Cartoon Network aired these commercials on practically every commercial break when the movie came out.
- A Sausage Party trailer spoils Carl's death.
- The nature of Puss in Boots (a cute little kitty who just happens to be a mercenary) is clearly meant to be a comedic twist, but the trailers for Shrek 2 practically made it the main selling point.
- TV spots for Shrek 2 also showed Donkey as a white stallion, though they didn't show Shrek in his human form.
- Shrek the Third's merchandise spoiled the birth of the Shreklings (Shrek and Fiona's children), as well as the Distressed Damsel princesses going Action Girl. Some of the trailers showed the princess' action sequence in its entirety.
- A French trailer for Song of the Sea spoils the plot from beginning to end, even going as far as to include the last shot of the film.
- Even the earliest trailers for Spider-Man: Into the Spider-Verse showed the Signature Scene of Miles finally taking his "leap of faith" and nearly mastering his powers. Granted, that plot point's a bit of a given, and the scene does showcase the best aspects of the movie, but it's still giving away a huge plot point. Most audiences didn't really mind, though.
- One of the trailers for The SpongeBob Movie: Sponge Out of Water clearly shows Plankton as Plank-Ton.
- The trailers for Strange Magic: The scene where Roland makes out with some kind of insect is frequently shown in trailers.
- The trailers for The Thief and the Cobbler had clearly shown the entire plot of the film which included the three golden balls being stolen from the kingdom, the climatic battle with the main villain One-Eye and Tak and Princess Yum-Yum getting married at the end of the film.
- A home video trailer for Titan A.E. showed the entire movie, start to finish, in order, including the final scenes of a new Earth being formed and the lead characters on it. As a result, some didn't bother to see the movie. Indeed, the movie didn't do well in theaters; this might have been a factor.
- Toy Story 3:
- Toy Story 3 had trailers that spoiled that the new toys were the villains. And while they never explicitly stated who the Big Bad was, many viewers were able to tell just from what was shown (and if not that, then from the other marketing). In the later trailers it almost revealed that the real plot was an emotional rollercoaster.
- The Japanese trailer was even worse about this, as is showed pretty much every main plot point.
- The trailer for Trolls advertises new songs by Justin Timberlake, who voices a character that does not sing at the beginning of the film. What are the chances that will change by the end?
- The trailer spoiled that Charles Muntz is the villain.
- Most trailers for Up were actually a remarkable aversion to this trope. Other than the presence of a talking dog and a floating balloon house, nothing else was really shown.
- Wallace & Gromit:
- A TV spot for The Curse of the Were-Rabbit spoiled that the were-rabbit is Wallace, showing fuzzy fur popping out of his trademark sweater-vest. It even had the announcer say "What started as a little mistake became a big problem" over scenes of the mind-manipulator accident and said transformation scene.
- The theatrical trailer was even worse. At one point it shows Wallace with long, fuzzy rabbit ears. Doesn't get more spoiler-y than that.
- This trailer for Elisabeth blatantly shows the Mayerling Waltz. (Sisi's death doesn't count, because Lucheni says "I killed her because she wanted to die," some several seconds into the video.) It's not much of a spoiler for German/Austrian audiences, or people familiar with that era, because both the Empress' Impaled with Extreme Prejudice and the Crown Prince's Murder-Suicide is well-known. Outside of that, though, viewers caught unaware (especially those who have issues with viewing gun violence and suicide) are in for a potential shock. The trailer narrator even says,"It's more than a show, it's history."
- New Broadway shows often fall into this when performing songs during televised events, such as Today or the Macy's Thanksgiving Day Parade. Naturally, they'll choose to show off with a big or emotional production number, which may be very late in the show or even the finale. One example: The Prom is about a group of Broadway stars rallying for a high school lesbian couple to attend their prom. The number they usually perform for sneak peeks? The finale where the stars put together an inclusive prom for the school, where the girls proudly dance and have The Big Damn Kiss.
- A commercial for Cats prominently features the scene where Grizabella ascends to the Heaviside Layer.
- The website description for the London transfer of Be More Chill outright states that the SQUIP's true goal to Take Over the World.
- Parodied in The Demented Cartoon Movie, which opens with a mock trailer that does the exact opposite: it doesn't reveal anything about the movie it's advertising because it's heavily censored, and parts of it have been replaced with stuff like [Dialogue Missing] and [Title Missing].
Why are they all missing?
- In the Black trailer, which serves as an introduction to the character Blake Belladonna, the background music is "From Shadows", a song with lyrics about people suffering Fantastic Racism from humans and planning their revenge. One of the plot points in the series involves the Faunus race being subjected to prejudice by humans (the lyrics in the trailer include "treated like a worthless animal", to make it clear the song is about them). This ends up spoiling, or at least strongly hinting at, the fact that Blake is actually a Faunus disguised as a human, which is a twist revealed in the second-to-last episode of Volume 1.
- Another example: The Japanese Volume 3 trailer reveals every death and tragedy in the season, which took away much of the impact present in the original release; where everything went to shit with very little warning.
- Crunchyroll's ads for Volume 4 blatantly spoiler the villain that was revealed right at the end of the Volume 3 finale. It also spoilers that Ruby, Jaune, Nora, and Ren are traveling together.
- Parodied with the second trailer for The Way of the Metagamer 2: In Name Only, which intentionally reveals many, many plot twists.
- The trailer for the Homestuck video game kickstarter was aimed entirely at people who had already read the comic, so it contained spoilers for ALL the major updates as of the time it was made. At least the spoilers were flashing by so fast that new readers missed half of them and didn't know enough to recognize many of the others as spoilery.
- Persona 4TW Add-On lampshades the fact that the big plot 'twist' was given away on the cover:
Shadow Labrys: Labrys being a robot? Heavens to murgatroyd. What a complete and utter surprise...or it mighta been if it hadn't been given away on the ***ing box art...
- The trailer for That Guy with the Glasses' two-year anniversary special Kickassia had Spoony shouting "Oh my gosh! It's 3D Lee!" And the final trailer for their fourth anniversary special To Boldly Flee spoiled The Nostalgia Critic absorbing the Plot Hole.
- More like "the preview always spoils", unless the YouTube user has found a way to muck with the video thumbnail of a movie, YouTube will default it to the middle of the movie. If it's say, for a race and the course is known, you can tell at least midway who's winning. A variation: if you're going to watch a comedy video, you'd do well to avoid looking at the top comments. They almost always contain the funniest jokes.
- Parodied by CollegeHumor in the hypothetical "If Movie Trailers Ruined Endings", where the trailer guy goes out of his way to spoil the endings and plot twists. Warning: spoilers for Fight Club, The Usual Suspects, and Reservoir Dogs. Also includes references to Luke, I Am Your Father and All There Is to Know About "The Crying Game".
- After part one of the Tabletop episode in which the horror RPG Dread was played, a "next time" trailer manages to spoil every major plot development in part two, rather ruining the tension that had been built up in part one.