Movie trailers are known to mislead, but sometimes they go in the opposite direction, giving away key plot points and twists (and sometimes what would have been a Twist Ending). The odds of this happening increase for the commercials aired after a movie's opening weekend.
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 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, 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.
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. Some shows give a "Next, on X:" segment, spoiling 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.
Can lead to Trailer Joke Decay. See also Spoiler Opening and Late Arrival Spoiler. Compare The Namesake, when the title itself may be a spoiler. Or just see Spoiler Title.
Almost anytime that a major character is going to die or there's going to be a major status quo change to The Verse the comic companies are bad about this and will announce the death of the major hero to the major news outlets in an effort to drum up sales. To make matters worse this is usually used to amplify stories that are already a bit spoiled by Tonight Someone Dies or Oh, and X Dies.
The Clone Saga was such an example. 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
In 2011 Marvel Comics has been 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 alot 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. The only reason it didn't make any sort of media splash was because DC Comics one upped them the day before by announcing that they were relaunching their entire line of comics this September.
In 2013, DC Comics spoiled the death of Damian Wayne, the most recentRobin, days before it actually occurred in Batman Incorporated. This of course gave media outlets ample time to cover the story.
The Darkwing Duck story "F.O.W.L. Disposition" has Steelbeak doing an Enemy Mine with Darkwing after F.O.W.L. goes too far. Whether this is believable or not in the first place is debatable, but every single cover shows him as an evil figure, including the back of the trade.
It's a pain trying to find a surprise twist in Fan Fiction because so many writers think "summary" means "tell me everything that happens in the story".
This is especially the case when the surprise twist involves a ship. We get such hilarity as "Which girl will Harry choose? Harry/Hermione."
Granted, not including the pairings in the summary is practically asking the rabid shippers to flame you. And few fandoms have more rabid shippers than Harry Potter, where even crackiest of crack ships have rabid fans.
A summary that spoils the work it's based on from a piece of Death Note fanfiction: What if Rem never finished writing L's name in the Death Note? How will L cope with the loss of the only father figure he's ever had in his life? May contain spoilers. Just in case there was any doubt at all, the story is titledWatari.
Fandom specific: There are quite a few Total Drama Island fan competitions on fanfiction.net involving the original cast that say who the final two are in the "characters" tags.
The very fact that "character death" is considered a required warning for fanfiction means that any fic that features a character's shocking death, or a character thought dead who turns up alive, will be spoiled from the get-go based on whether there is a warning or not.
Along the same lines of "character death" warnings, authors will often also put warnings for sex-related plot points such as Slash Pairings, Lemon scenes, and Mpreg. While this is nice for readings that find these things Squicky, it makes it all the less surprising when Mr. X and Mr. Y reveal themselves to be gay, have sex together, and end up pregnant as a result.
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.
Film - Animated
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.
In Shrek 2, the nature of Puss N Boots (a cute little kitty who just happens to be a mercenary) is clearly meant to be a comedic twist, but the trailers practically made it the main selling point. Shrek 3 's merchandise spoiled the birth of the Shreklings (Shrek and Fiona's children) and the dragon/donkey hybrids (Donkey and Dragon's children), as well as the Distressed Damselprincesses going Action Girl.
TV spots also showed Donkey as a white stallion, though they didn't show Shrek in his human form.
Coraline. The trailer shows the scene where the Other Mother gives Coraline the buttons and sewing needles.
The trailer for Up spoiled that Charles Muntz is the villain.
Most trailers for Up were actually a remarkable aversion to this trope. Other than the prescience of a talking dog and a floating balloon house, nothing else was really shown.
In Paranorman the trailer spoils nearly every joke, even if it does not spoil the plot for the most part.
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.
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.
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. Why bother to see the movie?
Chicken Run: As shown in one of the TV commercials, the chickens manage to escape by making their own pedal powered plane!
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 did they have to show she's recognized as such?
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).
The Lion King's trailer spoils most of the plot. Movie posters and the DVD cover show Mufasa's ghost as well.
The DVD cover for How to Train Your Dragon not only shows Hiccup riding Toothless, but Astrid riding her new dragon as well. Astrid and the other young vikings didn't ride dragons until the Big Damn Heroes moment in the film's climax.
The trailers for the sequel to Hoodwinked! 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 creepygrins saying: "You'vebeen hoodwinked!Too!"
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 TV spot for Wallace & Gromit: 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 toy commercials for the Bionicle 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...
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.
Many RPG adventures' cover art, seeking to entice buyers with action scenes, inadvertently spoil the nature of the scenario's Final Battle or a major mid-story menace.
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].
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.
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.