The entire Dragon Ball metaseries (all dubs) plays with this trope, particularly at key moments. It's done as a ratings ploy: if you want people to watch, tell them exactly what they'll see. Why would I want to miss Trunks kill Frieza in Dragon Ball Z? Also, the manga necessarily spoiled the anime. Common to any anime closely following a manga.
In Japan it was assumed that the entire fan base was reading the manga so they would just tell them what happens in the next episode either through the title or through the preview.
The trailers for the Battle Angel Alita OVA summarize the entire episode, up to and including the defeat of the major villain.
The English-language release of Vol. 30 gave away a major plot point (who gets the final antidote) in its choice of picture for the "In the next volume" page at the back. With a little thought, it's easy to deduce what happens. It's something of a moot point, since it's suggested that Sakura could have made it back to the Sand Village to make another antidote (the thing paralyzes instantly but takes 3 days to actually kill someone) if not for Chiyos's Heroic Sacrifice, which the preview for the corresponding anime episode spoils.
Naruto's Volume 42 preview shows Sasuke saying that the Mangekyo Sharingan 1) enables the user to control tailed beasts, 2)causes blindness.
The preview for the fourth uncut DVD collection reveals nearly all of the matchups in the preliminaries and, most egregiously, features a shot of the winners gathered together.
One news post about Naruto chapters 430-434 (mislabeled as 425-429) mentioned that "when one of Naruto's comrades intervenes, more tragedy may be in store." While they did warn about spoilers, the event in question (Hinata's confessing her love to Naruto and trying to defend him against Pain) doesn't happen until Chapter 437.
Promotional material for The Last: Naruto the Movie, which takes place between the penultimate chapter of the Naruto manga and its epilogue, started showing up four months before the manga even ended. As such, it ended up being the first place to reveal that pretty much everyone lives when it showed off their character designs, and because a good deal of the trailers and art showed off the romantic movie's Official Couple, gave away that Naruto and Hinata end up together.
Viz Media is incredibly bad with this. . In their translation of One Piece, they also showed the exact page where Luffy defeats Captain Kuro.
If you look closely in the background on the preview in Volume 66 for Volume 67, you can see Trafalgar Law on Punk Hazard.
In a strange case, the Post Episode Trailers on the first three episodes of Yes! Pretty Cure 5 (as well as the promo trailers, which focused a lot on the first episode) each showed part of the Transformation Sequence of the girl who would transform for the first time in that episode. So in the trailer previewing episode 5, the conspicuous absence of a scene spoiling Karen's transformation was a spoiler in itself. (Or at least, in hindsight, it should have been.)
On the other hand, although the Pretty Cure fandom was more-or-less unanimous about CureSunshine's identity, there was still some suspense to be had in-show...until the trailer for Episode 23 killed it: the preview footage consisted almost entirely of Itsuki transforming into said Cure.
The infamous "Malay dubs" of Transformers Headmasters would often have major spoilage IN THE TITLE ("Ultra Magnus Dies!" Wonder what that episode is about). And one particularly amusing spoiler from the narrator in one episode:
"Will Scorponok return? Of course he will."
The trailer for the English release of SHUFFLE! does a good job of keeping the secret of who the Unlucky Everydude ends up with, until you realize that it's playing Asa's theme music. This is a bigger hint than it would usually be, since Asa is usually pushed aside until the viewer is hit in the head with her surprise victory, even left out of most plot summaries! Someone is going to put two and two together.
Mobile Suit Gundam SEED spoiled the death of a certain character by having one of the tracks on the official soundtrack being titled "(Character)'s Death".
The tendency for trailers to spoil is parodied in Sayonara, Zetsubou-Sensei. The plot summary on the back of the first volume of the manga appears to be spoiler laden, until you learn it has nothing to do with Zetsubou's actual plot. Likewise, the end of the Magical Girl parody features a "spoiler" filled "Coming Next Episode" sequence, revealing (among other things) that Nozomu's mask was made of cardboard, and that he hasn't actually been defeated. Naturally, the real next episode is a return to the show's usual format.
D.Gray-Man. All the volumes end with a panel from the next volume. This panel is often a spoiler- let's see... volume 8 shows Allen's destroyed Innocence reforming and volume 11 shows that it takes more than an iron maiden to kill Krory.
Digimon Xros Wars usually spoiled its content by releasing preview images of the next part of the story before the previous one aired.
Before the Seven Kingdoms story arc began halfway through the series, it was revealed that Taiki, Kiriha and Nene were to get new clothes, join forces and their Digimon will gain golden-armored forms.
Before the end of the series, plenty of previews of Digimon Xros Wars: The Young Hunters Leaping Through Time were released, confirming that Damemon will be revived and become Yuu's partner for good (hasn't happened yet), that Shoutmon being killed off in episode 53 wasn't going to be permanent, and that in Young Hunters all characters were take the bus except for Taiki and Yuu.
The next episode previews at the end of each episode of Digimon Frontier usually spoiled the most important plot points of each episode. And if one simply chooses not to watch those, the episode titles themselves would be more than happy to do the spoiling in their stead.
The Movie could be considered a spoiler in this manner for Digimon Adventure 02 in one case (two if you saw it on it's American release date of October 6, 2000). The biggest spoiler was the Golden Digi-Eggs, which would not appear in American airings for over a month after the movie was released (November 18, 2000). Viewers who saw the movie on the American launch day were doubly spoiled, as they also spoiled Raidramon who would not be shown until the very next day.
The trailers that came out before the second season spoiled a lot of the stuff in the beginning of the season. Then again, none of it would be news to anyone who'd read the original sound novels.
The next-episode trailers in the first season are usually too vague to give much away, but one of the Atonement-arc trailers includes the phrase "What I opened was a scrapbook of deception", which spoils an upcoming plot point.
Volume 2's preview spoils Nina and Alexander's unfortunate fate.
The trailer they made for the Season 1 boxset of the 2003 anime adaptation shows various clips from the first 26 episodes. The clips they use to end the trailer on? Maes Hughes' death.
Although Fullmetal Alchemist: Brotherhood usually avoids this, the trailer for episode 50 completely ruins episode 49's Cliffhanger of Mustang's allies about to be shot dead by, after a pause of about half a second, showing many of the "shot" characters perfectly fine in the On the Next trailer.
The manga preview for Volume 27 spoils Orihime being abducted, while at the end of Volume 26, she had only just encountered Ulquiorra.
The summary of Volume 15 of the Bleach DVDs spoils Aizen being the Big Bad, by referring to him as a captain who was thought to be dead, screenshots showing him clearly alive and the cover being of him after The Reveal.
The trailer for Episode 224 of Bleach clearly shows Momo helping Rangiku.
The preview for Volume 24, while questioning what the outcome of the battle against the Arrancars will be, shows Renji, Hitsugaya and Rangiku winning their fights.
Shonen Jump sometimes spoils plot points in its previews for the chapters in the next month's issue.
In the December 2007 issue, the Bleach preview mentioned "reinforcements from the Soul Society" coming to help Ichigo against the Arrancar, and it was still several chapters before Hitsugaya's team arrived.
The June 2009 issue suggests that the next issue's Naruto chapters would feature "the confession you never thought you'd hear", and it isn't hard to figure out that they're referring to Hinata.
The July 2009 issue features a preview frame in Bleach showing Dordonii's defeat.
Every "Next Episode" trailer in Mobile Suit Zeta Gundam does this. Watch the trailers and you need never be surprised by a plot twist again.
The trailer for Episode 405 of One Piece spoils almost the entire episode by showing shots of Kuma lunging after each of Luffy's crewmates, and his saying that he can't save a single friend. And there's the title: "Disappearing Crewmates! The Final Day of the Straw Hat Crew!"
The English Trailer for Ghost in the Shell starts with revealing the mystery the entire plot is about.
The trailer for Full Metal Panic!: The Second Raid at the end of "Episode 00" spoils that Gauron survived the explosion at the end of the first season.
Completely averted by the back cover of the final volume of Death Note, which, instead of giving a brief plot summary, simply says "The battle ends here!"
Mostly the same with the preview for it in Volume 11, although if you think about it, showing Light's watch could be considered a minor spoiler.
YuYu Hakusho's season four boxset mentions Yusuke dying and coming back with the help of his demonic genes.
You can always tell in InuYasha when a character will be making a "surprise" return or InuYasha will be going through his monthly transformation into a human by the pictures on the chapter covers (and sometimes the volume cover).
The first Japanese volume of the Fist of the North Star manga ends in the middle of Kenshiro's final battle with his rival Shin. Yet, the cover of the second volume spoils the outcome of the fight by depicting a dying Shin leaping to his death.
The preview trailers and posters for the 2007 Fist of the North Star movie Legend of Raoh: Chapter of Fierce Fighting spoils the fact that Raoh dies. Considering the movie is an adaptation of a key story arc in a 24-year-old manga, this is a combination of Late-Arrival Spoiler and It Was His Sled.
Trailers for various iterations of Neon Genesis Evangelion contain ruthless spoilerage. An ADV trailer briefly showed Misato and Ritsuko's death scenes from Episode 25, and a "next episode" trailer spoils Rei II's death; virtually every frame of the Manga Entertainment trailer for The End of Evangelion showcases massive spoilers, among them the invasion of NERV HQ, the Misato-Shinji kiss scene, Asuka's death scene, and the appearance of GNR; even an early Japanese trailer for Death & Rebirth/EoE features a voiceover spoiling Rei's betrayal of Gendo. Admittedly, EoE has so many turns that it would be extremely difficult to have a trailer that spoils nothing whatsoever, but a series of Japanese TV spots seems to have figured it out (and encapsulated the general spirit of the movie into 15 seconds, to boot:)
As awesomely disturbing as the DVD menu for End of Evangelion is, it too is guilty of plenty of spoilerage as well most notably Asuka's death.
The preview for episode 23 of Bokurano spoils the fact that Machi is the next selected pilot.
The DVD menus for the newest US release (Remix, I believe) of Cowboy Bebop do this terribly. The opening menu sequence on the first disk features the scene right before Spike dies.
A common trailer for the show clearly shows Wen holding a gun and with a hole in his head which probably made it blatantly obvious who was the real villain of the episode long before anyone even watched it.
The English trailer for Kiki's Delivery Service shows the climax of the movie: Kiki saving Tombo after regaining her powers.
This trope, together with Trailer Joke Decay, was discussed in The Dissociation of Haruhi Suzumiya (the ninth novel), when Haruhi decides to make a sequel to "The Adventures of Mikuru Asahina". She talks about how much this annoys her and decides to avert this by producing the trailer before even starting the filming of the actual movie.
The main Japanese Trailer of Howl's Moving Castle reveals the true form of the Witch of the Waste, but does not say that it is her.
The Next Episode previews for Yu-Gi-Oh! 5D's has on one or two occasions spoiled certain things. Episode 10's preview ends with Yusei holding up the Rubble King card (the last card he uses against Takasu) and episode 12's preview spoils the end of the Yusei/Ushio duel by showing Goyo Guardian being destroyed by Turbo Warrior.
The Next Episode preview for episode 53 spoils the outcome of the Crow/Bommer duel. The very last scene of the preview shows the exact scene in which Earthbound God Chacu Challhual is destroyed.
The previews for the film adaptation of Osamu Tezuka's Metropolis showed scenes from when Tima realized her true potential - to destroy Metropolis.
And not only have the trailers given off the climax, but the DVD covers as well!
The preview for Volume 6 notes that "all (Mashiro and Takagi's) dreams may go up in smoke when one member of the team can't take the pressure," and it shows Mashiro collapsed in his office (Granted, this is foreshadowed).
The preview for Volume 12 asks whether Mashiro and Takagi entering a contest will lead to strife between the two; in the volume, things come to a head and Mashiro and Takagi, for the second time, seem poised to part ways. Interestingly enough, the accompanying image subverts this; Takagi punching Mashiro seems like a spoiler of the conflict to come, but it actually takes place as they reconcile. The summary on the back also strongly implies that Mashiro and Takagi save PCP from getting cancelled, but fail to get an anime.
In Mai-HiME, Episode 15's trailer, after a cliffhanger involving Mai's apparent death, contains a brief shot with Mai in the background while Yukino is ordering food and drinks at the Hime Sentai's karaoke party.
The intro of the first season of Pokémon Black and White had the Pokémon not covered in blacknote Unlike Best Wishes., thus revealing every Pokémon in the intro.
Merchandise and magazines tend to spoil future captures and evolutions for the cast. One notable reveal, however was that Ash's Frogadier will evolve into Greninja and Greninja will resemble its trainer. While the reveal in the Japanese magazine Corocoro teased the reason why, it was outright stated on the Official North American Pokémon website a mere couple days later.note Due to the fact there's a 2-3 month gap between a Japanese airing and an international localized airing of Pokémon episodes, the non-Japanese official sites do not normally report on future arcs to be aired in Japan. This only occurred because there was a reveal of new forms for Zygarde, hinting at a new Pokémon version for the 6th generation games.
In episode 19 of Eureka Seven AO, Christophe sacrifices himself to destroy Truth in a quartz explosion. The scene was incredibly well-done and poignant and the fans would've appreciated it far more if it weren't for the fact that the preview for the next episode clearly shows that Truth survived.
The North American DVD trailer for Slayers Evolution-R not only features footage from the final episode. It features footage from the last eight minutes of the final episode!!!
The trailer for Kuroshitsuji II has one shot—easy to miss, in their defense—that shows Ciel with black nails. This may seem like a minor detail to reveal anything, but the series establishes that the only characters with black nails are demons; hence revealing that Ciel becomes a demon.
Asura 's trailer reveal that Asura becomes a monk and Wakasa died, which is the last ten minutes of the film.
A relatively subtle example happens in the preview of the sixth episode of Girls und Panzer. The single frame shown for the preview, which otherwise merely announces the episodes' title, shows Mako flying away in a Black Forest helicopter, which spoils the unexpected source of help she gets to reach her hospitalized grandmother, as a result of Maho deciding to Pet the Dog.