CLANNAD has split-second clips in the background of the showcasing girls, which spoil certain future events and backstories.
The ending of Episode 13 of the Ace Attorney anime strongly implies that Miles Edgeworth is going to die, and in Episode 14 Phoenix outright states that he's dead. However, the latter episode also debuts a new opening in which Edgeworth is heavily featured, including shots of him standing in Times Square, pretty much spelling out that he isn't dead at all, just abroad. Sure enough, after a couple of quiet hints, Edgeworth shows up alive and well at the end of Episode 20. Of course, you'd know this if you'd played the games.
The OVA Ah! My Goddess Flights of Fancy has three massive spoilers in the opening. First it shows Hild, whose role as primary antagonist isn't revealed until partway through. Second it shows Belldandy being possessed by a devil, which is only revealed shortly after Hild appears. Third it shows Lind's two angels despite this being a major reveal during the climax.
The Akuma no Riddle OP is quite spoilery, and not just because it completely blows the Late-Arrival Spoiler that Tokaku will side with Haru. It also strongly suggests two things that happen much later: Sumireko is the final regular opponent (as opposed to Nio, who's sneaky about it) and Tokaku herself is still dangerous. The battle sequence also shows most of the assassins using the weapons they'll eventually fight Haru and Tokaku with — and yes, the fact that Shiena only gets attacked should tell you something, as should Hitsugi hiding behind Chitaru.
The ending sequence to Arata: The Legend shows Kadowaki prominently, putting his importance to the plot out in the open while displaying him and the Hayagami he eventually gains in the same way that other Hayagami and their Shou are shown.
In Azumanga Daioh, Kagura is all over the opening, making it obvious that she'll eventually be important despite her slow start. (There are other spoilers — in fact, most of the scenes in the theme end up happening — but they're all too weird to really give anything away.)
In Basilisk before the opening credits they show the scroll with the names of dead people crossed out. When Tenzen apparently dies at the end of an episode, his name does not get crossed out for the next opening.
BECK: Besides showing a few characters that don't appear for quite a few episodes, it also reveals that the band was "made to hit in America!" The idea of the band going for an overseas market doesn't come up at all until more than halfway through the series.
Black Bullet zig-zags this. The first few episodes use a Title Montage, with the real intro only appearing after a few key plot elements shown within it (such as Rentaro's bionic enhancements and supporting character Tina Sprout) have been introduced. However, said intro depicts Rentaro fighting with Kagetane, who he supposedly defeated for good two episodes previously, spoiling the fact that he's Not Quite Dead. Sure enough, he returns in Episode 11... only this time he's working with Rentaro to help defeat Aldebaran.
Black Butler: the second opening shows Angela with a gun and then Ash walking among fire, implying both to be villainous. They are, and this is before they are revealed as such.
Averted as the second opening only showed Diva from the back, keeping her face secret as she had not yet been revealed as Saya's twin.
The 4th opening falls squarely into this, revealing Hagi's wings long before they're actually shown.
It takes a really sharp eye to see in the first opening any hint alluding to Diva's pregnancy in the second half of the series.
The first opening of Bobobo-bo Bo-bobo spoils several characters' Heel Face Turns, though the inclusion of Captain Battleship among the main cast when he never actually joins up may have been an attempt to partially avert this. (of course, few people watch Bo-Bobo for the plot)
In Campione!, Liliana first shows up as a vassal of the Marquis Voban, and her HeelFace Turn is later. However, both the OP and ED display her as a member of the harem, making the outcome of those events rather foreordained.
The opening of A Certain Scientific Railgun includes some fairly important things that don't show up until the end. There's not enough context to figure anything out just by seeing it in the opening, though.
Claymore is a rare case that shows a battle that never actually happens and simply cannot happen (Claire vs Teresa) because it's symbolic for Claire choosing her own path rather than copying her step-mother Teresa. It also shows various other Claymores but makes it vague which ones are important friends/enemies and which are basicly Cannon Fodder. It does spoil however Priscilla's awakened form.
The other openings tend to spoil some stuff. The second version of the second opening has a Blink-and-You'll-Miss-It scene where Cornelia's Gloucester thrusts its lance. The reflection on its lance shows Marianne when she was killed. This is actually subverted at the end of the season- it was revealed that she didn't know who killed her. There's also a lightning-fast cameo of Cyborg!Jeremiah, several episodes before this comes up in the plot.
The second-version-second-OP in the first season goes even further than that: around the last 10 seconds or so of it, there's not only a shot of V.V., there's also one of the nun who gave C.C. her Geass, and, later, her Code. Of course, you virtually never notice it seeing as those shots are in the form of semi-transparent image flashes in front of the backdrop of Lelouch dramatically sprawled out in from of that inferno.
The first OP of R2 spoils Charles's Geass and Jeremiah returning, along with a shot of a space full of thousands of a million Zeroes.
The second R2 OP shows a brief shot of a C's World mask and C.C., good ol' Charlie, V.V. and possibly Marianne standing on the cliff with all of their long and shining locks blowing in the wind. The fact that Shirley was missing from that opening gives you a pretty good hint of what happens to her in the episode where it's first used.
You can see Schneizel in the first opening for a good four seconds before the last shot of Lelouch. Even though Schneizel doesn't show up until the last few episodes of the season. And the fact that they were the last two characters shown with the same background drop, implied to be disdainfully/mockingly looking at each other giving away that he's a major enemy to Lelouch... in the second season.
The opening of Coffin Princess Chaika spoils, or at least heavily foreshadows, the existence of the other Chaikas, even showing Chaika casting aside a blue veil in an identical manner to Layla/Blue Chaika during The Reveal. It also shows an injured Toru falling into the water in the sinking Soara Fortress, and Chaika diving in after him, as happens in the final episode.
The opening for Cowboy Bebop suggests that Ed is going to become a regular cast member, seeing as how she's heavily featured in the intro even though she doesn't appear until a few episodes in. The same holds true for Faye who is antagonistic with the crew for the better part of two episodes before finally joining.
Spoiler ending in Daily Life with Monster Girl's case. With the credits showing the MON squad, revealing that there's a special monster squad, who clearly have a connection to Ms. Smith, and don't show up till much later. The opening itself is also mildly spoilery, as it reveals all but one of the girls who end up in Kimihito's harem.
The First-Episode Spoiler of Darker Than Black is unlikely to come as a surprise if you pay much attention at all to the opening, but it avoids anything more serious since it mostly just consists of Hei looking cryptically badass. There is, however, some foreshadowing, in the form of Amber's silhouette. And that Hei does not like Amber if you pay attention to his facial expressions in the end. The sequel, however, completely averts this and has VERY Bait-and-Switch Credits.
The opening and ending of The Daughter of Twenty Faces feature Chiko alongside Shunka and Tome, neither of whom join the main cast until around episode 7. More importantly, the title and ending sequences completely downplay the members of Twenty Faces' crew, who initially seem like major characters, but are violently killed during the Wham Episode.
See that goth in the opening of Death Note? Her name is Misa. She'll be important later.
Also, a brief shot of L's face is shown, even though it isn't shown in the show for the first several episodes.
L is also shown on the cover of the second volume of the manga, which you most likely would see before his actual reveal.
The very last image in the second ending sequence is a shot of Light's corpse and a few flashed images before that show close-ups of Light's corpse's eye, with Ryuk reflected in it. Granted, it may be more foreshadowing, as you're not likely to figure out that's what it is among the other weird crap.
Don't forget the second opening, which shows shots of all of the new major players in the latter half of the story a good few episodes before any of them are introduced.
Third opening showed what Skinn Bolic was capable of doing in combat. And Allen's broken arm, Bak, For's fight with the lvl3, Kanda's weapon upgrade, the giant-combined-akuma-thing, Chomesuke's human form, the ark, Crown Clown and Lenalee's short hair.
The fourth opening does this the most: showing the Generals with their Innocence, Jasdevi combined, Tyki's other form, the Fourteenth Noah as well as the Score, Lvellie as well as Howard, Chaozii's Innocence...in short, everything besides a certain 'beer belly punk'...
Diebuster has one, although it's easy to miss: right at the end of the opening sequence, Nono flies away — which she can do because she's a Buster Machine.
A spoiler ending. The very same episode that announced the existence of the new Chosen Child and Digimon also debuted the second ending theme, the animation for which very obviously portrays Taichi's little sister and Vamdemon's Right-Hand Cat among the good guys. Not only that, it spoils what the unrevealed Ultimates (and Megas, a level not known to exist yet) would look like.
The English dub opening that added Kari also had a pair of glimpses of Angewomon, introduced much later, and MagnaAngemon, who was the second to last Digimon introduced in the series. Several episodes after this opening, Izzy refers to (basic) Angemon as Patamon's fully evolved form. This was the same episode that introduced the concept of Megas, so clearly there was more to be seen.
The first Japanese opening also did this with Angemon. When Devimon appears and becomes a major threat for several episodes with Patamon being the only mon that hasn't reached Champion level yet, a viewer can easily guess that he eventually digivolves into Angemon and defeats Devimon. Since Devimon is a (comparatively) weak foe, this is understandable.
Digimon Adventure 02's second Japanese ending, which started at the time of Paildramon's introduction, was particularly bad about this. It shows Silphymon, Shakkoumon and even Imperialdramon, in both Dragon and Fighter Modes. One episode later, the opening was revised, which in case you hadn't already seen the ending, shows the then-unintroduced Silphymon, Shakkoumon, and Imperialdramon Dragon Mode.
Zig-Zagged in the first Japanese opening, where the three main characters are joined by another three shadowy figures. Two of them are obviously Juri and Hirokazu, but the other one is very indistinct as the writer hadn't yet decided who that was going to be when it came for the sequence to be animated. This other one eventually became Kenta in the second Japanese opening.
According to Chiaki Konaka's website, only Juri and Hirokazu were going to become secondary Tamers later on in the narrative. But in an earlier part of the first Japanese opening, there's a shot of a ton of kids holding D-Arcs, including Shiuchon. According to her profile on Konaka's site, Shiuchon is a case of Throw It In!: because she was already in the shot, they decided to make her a Tamer later in the series.
While Kenta and MarineAngemon are clearly shown at the middle and end of the second Japanese opening, the shadowy figures occupying the same position in the first Japanese opening don't resemble them in the slightest. Konaka has stated that it originally wasn't Kenta in the OP, which is why he's way in the background, instead of in the foreground with the Power Trio, Juri, Hirokazu, and Shiuchon. Kenta was given his partner at the last possible minute, even after it was planned for Juri's partner to die.
The first Japanese opening shows not only Guilmon's, Terriermon's, and Renamon's Champion forms, it shows their Ultimate and Mega forms too. Then around the time of the Digital World arc, the second Japanese opening gives away Impmon digivolving into Beelzemon and pulling a FaceHeel Turn. To make things worse, it also spoils him going into Beelzemon Blast Mode, which also spoils him rejoining the good guys and getting a Redemption Promotion, since he doesn't have wings and an Arm Cannon while he's a villain, and Digimon only change that drastically through The Power of Friendship.
Digimon Frontier's second Japanese ending was spoilerific. It begins playing at the end of the episode revealing Duskmon is a human, and shows Kouichi as a member of the group, his natural Darkness Human and Beast forms, AND Takuya and Kouji's upgraded Digivice designs that weren't introduced for another eight episodes.
Italian dub opening clearly states that 1) the hero will discover his Mysterious Past ("who are you? Goku, you don't know, but you'll soon find out"); 2) The Hero Dies ("and then you'll disappear"); 3) the hero will come Back from the Dead ("but you'll return and (...) you'll defeat darkness")
Dub theme contains several major spoilers toward the end. Three Super Saiyans are shown, including one who hasn't even appeared yet (Future Trunks, fighting Freeza, who also hasn't appeared yet — and in his final form to boot!). The worst one is Vegeta's inclusion in the final group shot, which makes it obvious that he'll end up a good guy; this is a spoiler you can understand almost right away, but which takes a long, long time and a lot of character development to happen.
It isn't just Trunks fighting Freeza - it's Trunks slicing Frieza in half, which is part of his death scene.
Later, in Dragonball Kai, the openings spoil numerous things, including the Saibaimen, Vegeta and Goku fighting, Namek, Freeza, Goku fighting him as a Super Saiyajin, the Androids, Cell's forms, several people going Super Saiyajin, and the fights between #17 and Piccolo and #18 and Vegeta. YMMV, given the series' age.
The French intro is... a peculiar case. Despite it being a sugary-sweet song about Gohan, the clips hardly mesh with the lyrics — except for the part when it refers to Goku in a past tense and shows him with a halo. But, it also has clips of him being alive, albeit battered, bruised and bloody, thereby revealing his return and his near-death. The entire intro consists of clips lazily taken from a couple of episodes from the Saiyan and Namek sagas, so it also shows Vegeta as an ape, the beaten Vegeta advancing on Gohan, the Ginyu force, Guru charging up Gohan's power, and the team looking for the Namekian Dragon Balls. And it was never updated, but at least this meant that it didn't spoil anything from the rest of the show. Nor did the closing, seeing as it was the same as the intro, only with credits.
The Spanish version of the French intro openly says "and his father is already dead".
The opening for the American-produced second season of Duel Masters gives away a large number of plot points, though in a manner that only makes sense once the viewer notices the pattern. It shows an established character that will represent each civilization, alongside a creature from that civilization that will be important later. Most glaringly, the final shot pans by Shobu's father and a Pyrofighter Magnus.
The first opening of Durarara!! shows us Celty's head. Surprisingly, it doesn't reveal the name of Seiji's girlfriend, at least in the translated credits.
The opening features Seleção symbols with Roman numerals one to twelve on them, supposedly referring to a speculated twelve agents working for this mysterious organization (Akira is IX). During this sequence, IV, V, and X fade away in that order.
Similar to the first example, there's a brief sequence in the first movie's opening that shows lines coming out of I's Seleção emblem and destroying II and XII.
The closing credits are perhaps even worse. They show Akira's paper cutout pointing at and shooting down "missiles" before they can strike. The same happens in the last few minutes of the final episode.
The first opening for Eureka Seven: Psalm of Planets - Days - features a call-ahead into the final leg of the series where Eureka and Renton have traveled over the Great Wall, under the layer of Scub Coral, with the Nirvash.
Fairy Tail has done this in both the anime opening and the manga splash pages. The anime's tendency of showing major fight setups, characters whose involvement you aren't supposed to know about yet, and hints towards certain plot twists and heel face turns is one of the great ires of the fandom. Although the plus side to it is that if you ever need a refresher, you can go through all the openings and to get a short summary of all the key twists and fights in the anime thus far. With the exception of the 1st and (maybe) 5th opening, all the openings have started airing in during the middle of the arc prior to the one they focus on.
The manga shows Wendy and Charle in two group shots of Fairy Tail mages before she joins them. The all-female one is understandable, but showing up in the background of a color pin-up of Erza and Lucy in swimsuits with Natsu, Gray, and Happy staring, with Wendy in her own swimsuit, trying to catch your attention by being in front of the title, more than three months before there was a hint of her joining was kind of suspicious.
The first opening doesn't even bother trying to hide Lucy's teammates, their powers, and her summons in both the opening and the ending credits, most spoileriffically Virgo and Erza's super armor. It also implies Erza's involvement with Siegrain.
The second ending also shows Gajeel at the start of the arc before his first appearance.
The third opening focused almost exclusively on the arc after the one that was currently airing, thus spoiling a HeelFace Turn. It also subtly hints at the true (plot important) nature of one character's magic quite a ways before it's revealed in the story proper.
The fourth does the exact same thing, spoiling Laxus turning against the guild, Gajeel returning as an ally, and astute viewers will note that Mystogan has the same hair and eye color as Jellal. It also shows Mirajane losing control of her powers.
It's bad enough that the fifth opening shows that Lyon and Sherry make a come back, and that Jellal isn't dead, whereas before it was kind of vague about the last one, but the ending actually shows Wendy and Mystogan together as a child.
7th Opening shows the characters alternate universe counterparts. It also opens with flashbacks of Natsu's childhood with his dead friend Lisanna, and ends with a shot of him desperately reaching for her alternate universe counterpart who's turns out to actually be the Lisanna he grew up with still alive.
And here comes the 8th opening and ending, in which we can see Queen Shagotte way before she actually appears onscreen, hints of the final boss of the arc, and the return of Mystogan. But the worst of all is the new ending heavily focused on Lisanna, who we barely see during the Edolas arc, and the friggin end of said arc, in which Lisanna run into her brother and sister at Magnolia's cemetery. Really ? Spoiling the very last scene of Edolas arc ? Really ?!What the heck ?!
The 9th opening shows everyone participating in the S-Class Trial (including Mest Gryder, who should have come from left field) and spoils Grimoire Heart getting involved, showing Hades and all of the Seven Kin of Purgatory. It also spoils Zeref's presence (though admittedly, seeing him for the first time, you wouldn't realise it's Zeref). The 10th opening spoils Laxus's return, Natsu's Lightning-Flame Dragon Mode, as well as Acnologia's attack.
The 11th and 12th were particularly horrendous, due to including characters that most fans will recognize ... First they show the Earthland versions of several prior characters who only appeared on Edolas, followed in the very next opening by post-timeskip versions of EVERYONE IN ORACION SEIS. Whether or not the first is truly an anime-exclusive is yet to be seen, none of these characters are in the manga ... yet.
The 13th and 14th openings spoiled several of the matchups for the Grand Magic Games, with OP 13 showing all the other qualifying teams (including Raven Tail and Fairy Tail B, who were supposed to be unexpected) OP 14 showing the combined Fairy Tail team, another spoiler.
The second opening of the Fate/stay night anime shows Gilgamesh raining swords on Saber some four episodes before his debut. Likewise, it also spoils Archer and Shirou fighting in Unlimited Blade Works... which doesn't happen in the anime or the route that the anime is mainly based on (Fate, and the route where they fight is Unlimited Blade Works).
The first opening of the visual novel shows Rin and Sakura depressingly turning away from each other. At first, it seems that it's because they're both Shirou's love interest in their respective routes, but it hints at their true relationship.
The ending of the Fate/Zero anime shows brief scenes of all the Servants' former lives. These are mostly vague (e.g. we learn that one character from Greek myth or history will be used, but not who that person is) but it's certainly possible to make a shrewd guess at Berserker's identity. Which is also the only one not revealed early on in the series. It's also possible to correctly guess their identities if you recognize what art pieces each of their sequences are based off of.
The first opening spoils Kiriya's insect-control-magic and Tokiomi's fire magic. It also foreshadows Assassin getting only a minor role in the series while every other Servant having a major role.
The second opening made it pretty clear of Irisviel's fate and Kiritsugu's. Not to mention showing glipse of Saber's motorcycle.
In Flame of Recca's opening, Koganei Kaoru is shown as a member of the main force of good guys. Too bad this didn't happen in the anime until close to episode twenty.
Flowering Heart's openings spoiled quite a few things, including Chess's human form, Ari's love triangle, and Syuel joining the girls.
The first closing spoils the outcome of the Elric brothers' encounter with Scar, with Al's broken armor and Ed's missing automail arm blatantly shown. Also, that scene doesn't even happen until the second opening and closings are in use!
Meanwhile, Fullmetal Alchemist: Brotherhood's first opening has three shots of Hohenheim, who, in the manga, doesn't really show up for another 10 volumes. One of the shots is even Hohenheim as a young slave. In their defense, Slave!Hoho is so ambiguous that someone who doesn't already know what it means will probably incorrectly take his appearance as a spoiler that Ed gets his arm back. The first opening also shows the alchemy diagram for human transmutation intact, which is a significant plot point much later in the series. As with Hohenheim's slavery, it's ambiguous enough that it's not a major spoiler.
The second Brotherhood opening spoils Gluttony's true form, Envy's true form, Ed's reunion with Hohenheim at Trisha's grave, Ling vs Wrath, and Mustang vs Lust. Though the actual Ling vs Wrath is very different from what was shown, and they spoil less than they would have because they aired it with episode 15 instead of episode 14.
The third Brotherhood opening shows Ling among the other Homunculi, as well as Pride attacking the Brigg's tunnel expedition team and possessing Alphonse. In fact, this opening also introduces Olivier Mira Armstrong and the Briggs army before we even officially see them. However, to their credit, the first instance isn't given very much attention and isn't all that noticeable, and the second one completely lacks any context (Not to mention how Pride's true self looks nothing like the other Homunculi, which could trick viewers into mistaking it for something completely unrelated. It also shows what looks a lot like Kimblee's final moment after "dying". That last one is probably the biggest spoiler, since the third opening made its debut months before the actual manga chapter where Kimblee dies in exactly that manner was published.
The fourth opening shows Greedling fighting Wrath and Fu and Lan Fan returning.
The ending is even worse, showing the transmutation circle under Grand Central made of the five labs being activated and every single HeelFace Turn for the rest of the series: Scar, Greedling, Darius, Heinkel, Jerso and Zampano (and to a lesser extent May, Marcoh and Yoki) are shown fighting Pride alongside the Elrics. Also both the opening and the ending show the reversed transmutation circle egregiously, at point on top of a map of Amestris for those people who didn't get the message. It also featured the Gate of Truth namely, Ed's dissolving, referencing something that didn't happen until the last chapter of the mangawhich hadn't even been published when the opening started airing. Of course, since that hadn't happened yet, it's very easy to take it as unrelated or stylistic.
The 5th opening shows who the 5th sacrifice is, Father's monstrous form, an his eye growing large, Al's body, a bloodied Wrath sitting as Scar removes his jacket to fight him, Ling in full Greed form atop a ton of wreckage, our heroes standing ready to fight including Sig, and Izumi (of course if you where paying attention you saw Izumi's feet with the Brigg's soldiers in episode 50), and-a bit of a more subtle one - Mustang's price, his sight.
Fushigi Yuugi showed all of the good guys together. In the series, they are only slowly gathered together, and during the process the bad guys try to infiltrate the cast with an agent. The series actually tries to pretend it's surprising the audience with this, but it's obvious to anyone who's been paying attention. It also shows Yui will turn bad.
Future Diary's opening is LOADED with spoilers, except they are either so subtle you don't notice them, Blink-and-you-miss-it, or completely incomprehensible for those who have not read the manga. For example, the scene of Yuno fighting herself and two halves merging together seemingly looks like it was thrown in to be frantic and creepy with a heavy dose of artistic license, but it actually foreshadows a major plot twist that occurs later on in the story.
Also applied to its closing, which reveals the identities of the diary holders, the reasons they are in the game, and Akise - especially Akise, albeit in shady lighting.
The second opening has a number of visual spoilers. And then there are the lyrics, which are in a hard to understand english, but once you understand it it reveals pretty much everything, including the time-travel plot, a LONG TIME before this happens in the story.
Monthly Girls' Nozaki-kun's opening briefly shows Nozaki lifting Sakura into the air which doesn't happen until the final episode. It's a Flashback of their first meeting.
The first opening of Golden Time portrays Kouko and Banri as a couple, even though their coupling doesn't happen until several episodes in.
The ending credits of GoLion (which became Lion Voltron) reveal that one of the members of the team dies and is replaced by the princess. The Voltron adaptation recycled this animation for it's ending credits as well, thus carrying the trope over to that version as well.
The intro of the Grisaia no Rakuen anime adaptation pretty much spells out that the Thanatos System is actually Yuuji's sister Kazuki.
Gugure! Kokkuri-san's opening credits rather prominently displays Shigaraki, Tama, Jimeko, Yamamoto-kun, etc... long before they are introduced in the show proper.
Although GUN×SWORD makes use of silhouettes, it still tips its hand at two points. For the first, the true appearance of the moon is visible (and unsilhouetted) behind the Claw long before it comes into play. The Evolving Credits actually cause the second, with three characters who presumably died in one episode not having their outlines shadowed before the next, indicating they lived.
The first season of the anime adaptation of Hakuouki shows Hijikata's transformation into a fury - a hugely dramatic moment which occurs only in the last episode of the season - in the opening credits sequence. The end credits sequence also pretty much spoils Kaoru's status as a villain.
Hanayamata: Five schoolgirls doing a fluid group Yosakoi dance in the opening pave the plot road ahead of one shy dreamy heroine who starts not even having a hobby to speak of.
Heroman: The first opening show Kogorr's One-Winged Angel, Will and Nick's shadows being Skrugg, Dr. Minami's mech, NIA.
The opening gives away the fact that Mion has an identical twin sister. If you pay attention, you might also notice that Shion, not Mion, is the one shown freaking out, which kind of hints at a plot twist in the fifth story arc (though it wasn't a particularly surprising one, anyway). It also spoiled the fact Mion has a Yakuza related tattoo.
Averted in the second season Higurashi No Naku Koro Ni Kai where though she features as a background character in the opening Hanyuu is edited out of the opening until she makes her main appearance later and they're slotted in.
The first shows, beside that Killua will join the the other main characters, Hisoka allying with Gittarackur whose true identity as Illumi Zoldyck is even revealed here, the other important participants and almost every examiner. And in the end, Gon's fighting Hisoka foreshadowed their battle in the Heavens Arena.
The second opening features Kurapika with longer hair, Zushi and Wing as well as the Nen and the Genei Ryodan! The second ending spoilers even the Nen types of the main characters, including Leorio's whose Nen type is revealed after the Chimera Ant arc.
The third opening is almost identical to the second, but the previous chorus part is replaced with featuring the abilities of the Genei Ryodan members. Subverted, since their abilities are shown one episode after this opening was shown on TV. However, it foreshadows Bonolenov's ability whose isn't revealed yet and Pakunoda's death.
The fourth opening shows Biscuit Krüger joining Gon and Killua, and the other important players of Greed Island, including the identity of the main Bomber Genthru. Then, Kalluto Zoldyck joining the Genei Ryodan, Razor and his 14 devils, Killua's and Gon's new abilities, and Hisoka as well as Goreinu and Tsezguerra's group joining the heroes for the dodgeball match.
The fifth opening shows the face of the Chimera Ant Queen and the Chimera Ant King in the first 15 seconds! It also features the appearance of Kite with his Nen, the other Hunters of the Invasion Squad and their abilities, some Chimera Ants, the Royal Guards, Gon's grudge towards Neferpitou and his transformation to his adult form (from the very end of the arc !), Netero's Guanyin and the foreshadowed death of Kite. The fourth ending on the other hand foreshadows even more Kite's death, and even Netero's, shown becoming a "star" in the sky. At least they didn't give away the appearances of Bisky, Pokkle, the Genei Ryodan, Zeno and Silva, the Chimera Ants who retained their memories, and... Komugi.
The fifth ending shows the two Chimera Ants who retained their memories and joined the hunters, Meleoron and Ikalgo. Komugi and her relationship with Meruem, too. One of Menthutuyoupi's abilites and Palm's Chimera Ant form.
Most main races in Initial D get spoiled in the opening from second season on.
Midway through the four season a mysterious driver in a Subaru curbstomps Takumi on his very favourite location which puzzles Takumi to no end...only that every viewer knows that the "mysterious" driver is actually Bunta, since he has been seen in that Subaru in the opening since the first episode.
Innocent Venus, somewhat. Jin appears with the same mask he wears prior to his Duel to the Death with Jo and some disembodied hands suggesting illusion, foreshadowing his FaceHeel Turn. The opening also strongly suggests that of the three leads, Jo will be the main character
It had a spoiler opening and a spoiler closing. The first opening sequence featured Shippo, Miroku, and Sango, long before any of them appeared. Sango likewise appeared in the end sequence. Sango is kind of an advanced case; She wouldn't actually appear in the series until after there were new opening and closing sequences.
The Final Act's opening spoils Sesshomaru getting a new sword and his severed arm back, Tessaiga's last two upgrades, Naraku's new incarnation, Byakuya, and Naraku's form after completing the Shikon no Tama.
Is It Wrong to Try to Pick Up Girls in a Dungeon?: The second season opening shows the Hestia Familia crew living in a giant mansion, quite unlike the tiny run-down house they had in the first season. They don't get this massive residence upgrade until the end of episode 4, when they beat the extremely wealthy (and extremely evil) Apollo Familia in a "war game" and claim everything they own as a reward.
The first opening shows major events in Phantom Blood: The Big Bad being shot at by the policemen with an image of Jonathan holding his father's corpse reflected in shattered glass.
The opening of Battle Tendency spoils Caesar's death. Specifically the part where Joseph ties Caesar's headband around his head while crying out towards the sky, and Caesar's last Hamon bubble (the bloody one). Kars' eventual immunity to sunlight is also foreshadowed, albeit more subtly since the shot of the sun rising behind his back is only a split second long. Some shots in the opening also foreshadow Joseph's stand for a split-second.
The first opening for the Stardust Crusaders anime shows Kakyoin and Polnareff fighting as part of the group, even though they initially appear as antagonists; additionally, during the shot of five stars shooting towards Dio's lair, a sixth one joins the group, foreshadowing Sixth Ranger Iggy. The second opening shows, although much faster, shots mirroring those just before the deaths of Avdol and Kakyoin, as well as the near-death of Joseph, and is replete with clock/gear imagery representing the fact that Dio's Stand can stop time. And there are many more split-second ones, such as Jotaro's hand moving during Dio's time stop and the World appearing in Star Platinum's reflection.
The opening for Diamond is Unbreakable likewise shows Okuyasu standing alongside Josuke, Koichi and Jotaro in spite of being introduced as an antagonist. Its second opening spoils the appearance of Echoes Act 3 and Yoshikage Kira's status as the Big Bad.
The first opening for Vento Aureo shows Bruno Buccelatti as a main protagonist, all the gang members and their stands, as well as Trish. The second opening shows Trish's stand, and more notably Giorno stabbing himself with the arrow and gaining a new power, which happens at the end of the Final Battle.
The opening of Jyu-Oh-Sei contains a lot of shots of Zagi, who is in the first episode but then disappears for half the series. This pretty much gives away that he's going to come back. Then it shows Thor when he grows up (which also doesn't happen until halfway through the series) holding a collapsed Tiz because she dies in the last episode.
Kamisama Kiss has both opening and ending spoilers. The show's intro rather prominently features Kurama, Mizuki, Otohiko and Sukuna before any of them show up. Also all of them had very antagonistic first meetings with Nanami but the opening shows them in friendly group shot which kind of spoils the fact that none of them remain enemies for more than one episode or so. The ending credits also spoil the fact that Kurama isn't a human but is actually a Tengu, that Mizuki can shapeshift into a white snake and what Tomoe's demonic bandit / wild fox character design looks like.
The merchandise line for Kamisama Minarai: Himitsu no Cocotama spoiled all six Cocotamas that Kokoro owns. The opening does this as well, and it also spoils the other Cocotamas that will be shown in the first season.
The opening of Kannazuki no Miko is effectively a 90 second summary of the entire plot of the series.
The opening of the seventh Kara no Kyoukai film prominently displays Lio's Stalker Shrine to Shiki and his drug producing operation, both of which are key reveals. However, it's presented in such a stylized manner that most viewers wouldn't pick up on it unless they'd already seen the movie before.
Averted by the Karin anime. Since the anime added a new character, Winner Sinclair, that was going to be a major player in the anime's plot, the OP only shows a conspicuous black silhouette of Winner whenever he would normally appear. This was naturally replaced with actual images of Winner when he was formally introduced.
The first opening briefly reveals Tsuna's X-Gloves, although they only appear for a second. After episode 20, the opening spoils Mukuro's gang, as well as Tsuna's Hyper Dying Will Mode.
The second opening for Katekyō Hitman Reborn! spoils nearly every fight and new attack in the next arc (although most of the spoiled attacks appear so briefly that you would have to be looking for them to notice).
The third opening spoils Xanxus's guns as well as Tsuna's Zero Point Breakthrough (though it doesn't spoil the ZPB Custom).
The fourth opening spoils the future forms of four of the main characters, the Box Animals they use, Yamamoto and Gokudera's new flame-based weapons, and Tsuna's X-Gloves: Version Vongola Ring. It also hints at the truth behind the Arcobaleno.
The fifth opening does the same thing as the second, showing key matchups and new techniques of every main character.
The sixth opening reveals Tsuna's completed X-Burner, as well as his new contact lens. It also reveals that Hibari and Ryohei traveled to the future, and that Tsuna's final opponent will be Genkishi. A remake for same opening, done for the filler Arc, included the real forms of all seven Arcobalenos.
The 7th opening has quite a few scenes of the entire Choice battle shown. Including Everyone's new Vongola Box animals, Yamamoto's new swords and the Real Six Funeral Wreaths.
Kekkaishi does this the other way around, the OP shows the 3 good guys together but in the anime the third person joins up later in the series only to be killed off soon after, surprisingly the OP never changes.
Well, there are some changes to accommodate major enemies or allies in the relevant arc. But for some reason they don't change anything else.
The OP also show Tokine using long, thin kekkai to skewer her enemies, a trick she doesn't learn until around the half-way point, and that she uses to compensate for not having Yoshimori's raw power.
Hikaru and Athena are shown fighting in the OP of Kidou Tenshi Angelic Layer long before it's even hinted that Athena will be more than an inspiration from one episode.
Those who have watched episode 2 and/or 3 of Kotoura-san would find the opening and ending spoiling. Both ends at a scene of Hiyori, Manabe, Haruka, Yuriko and Daichi standing outside of the ESP club room, which implies Hiyori would eventually be a member of the ESP Club. This further implies her issue with Haruka and Manabe—the biggest conflict in those two episodes— will resolve at some point.
The opening of Kurau Phantom Memory offers a glimpse of Christmas showing off some of her powers, which doesn't occur until much later in the series.
Little Busters!: If you pay attention (and are watching a version where the OP is translated, or speak Japanese), the second opening song pretty much describes exactly what's going to be revealed in the last couple of episodes - that most of the characters are going to die, leaving Riki and Rin on their own, and they're going to have to grow up and learn to stand for themselves to deal with it.
The Season 1 opening of Log Horizon pretty much spells out the entire plot of the season, from Demikas and Nyanta's fight early on to the goblin/sahagin invasion (doesn't happen until Episode 15), Lenessia's speech to the adventurers (doesn't happen until Episode 19), to Rundel Haus' death during the aforementioned invasion (doesn't happen until Episode 19).
Made In Abyss: The openings and endings briefly foreshadow many future locations (Seeker Camp, Idofront), characters (Ozen, Bondrewd), and events (Riko's swollen arm) but do so briefly and without any major reveals. The ending does however give away Nanachi's appearance and makes it clear that she's going to be one of the leads of the series. The ending starts airing on episode 2 and Nanachi isn't introduced until episode 10.
The second opening of Magical Girl Lyrical Nanoha Strikers spoils most of the matchups for the final battle (Teana vs. Wendi and Deed, Erio vs. Garyuu, Caro vs. Lutecia, Signum vs. Zest) with one notable exception; it shows Subaru fighting Nove when she actually fights Ginga.
Crossing over with Late-Arrival Spoiler, the opening for ViVid Strike! revealed that Ixpellia had woken up from her coma. Note that at the time that it came out she hadn't fully woken up in the ViVid manga. It also showed exactly where Fuka and Rinne would have their inevitable final battle, the training grounds at Hotel Alpine.
The first series of Magic Knight Rayearth manages to bizarrely combine this with a Bait-and-Switch Credits sequence, as the opening briefly but prominently features a scene from the climactic mecha battle with Zagato. There's little indication of mecha content in the series prior to them finally showing up besides this... but that's because they're just Macguffins who don't really play an active role in the story until this battle.
The opening spoils the appearance of Miyu in later episodes, in addition to showing a shadowy mysterious Otome, who might have not been prematurely identified as Mai, if not for her mitsu tomoe symbol from Mai-HiME. And of course, it also spoils the "will Arika get into Garderobe?" subplot of the first few episodes.
Starting with episode 16, a new set of opening credits appears... which not only reveals the (albeit well foreshadowed) major plot twist that hasn't happened yet, it also removes any doubt as to which side every character will wind up on.
Both openings, however, show a character in silhouette whose identity remains successfully hidden until the very last episode.
The main premise of the series Maria-sama ga Miteru is that of the soeur ("sister") system: characters choose lower classmen to be their Petite soeur. While the series tries to build suspense as to who will choose who, the opening sequence shows the pairs quite plainly. In the second season, there is supposed to be a rivalry between two of the girls, though we clearly see them as soeurs in the opening. The fourth season has an additional spoiler ending, since it clearly showcases who Yumi will choose as her soeur.
Halfway through Martian Successor Nadesico, a "real" pilot with all the appropriate training appears to pilot the main character's mecha. He is supposedly no longer needed. Said replacement is nowhere in the opening. The obvious happens.
Gai doesn't appear in the opening either, which seems very strange until the end of the third episode.
Rockman.EXE, in the Axess season, gains Soul Unison forms through the Power of Friendship. The OP spoils half of these forms from the outset, including those of characters apparently Put on a Bus, characters who currently hate him, and even characters who are villains.
Stream is better about this, going so far as to put Tesla Magnets in the villains' segment even though she eventually joins the main team. It does reveal that Yuriko is going to join the Net Savers, but only if you're paying attention.
Beast's suggests a link between Colonel and Iris (though this was obvious to anyone who knew anything about the X series), but essentialy lies by showing Chihiro and Blackbeard in the same sequence as the new allies.
The opening for Ryuusei no Rockman Tribe has Orihime in her geisha/priestess villain outfit, which she doesn't wear until the very end of the series.
Mekakucity Actors' opening credits are completely rife with spoilers. (Or at least spoilers for new viewers.) Especially the parts with Kuroha near the end.
Both openings of Mobile Fighter G Gundam show characters and robots before they appear, and the first indicates which antagonists of the week will become recurring.
The first Gundam SEED opening and ending spoils the fact that Flay joins the Earth forces (There's a lingering shot of her in a EF uniform), it spoils the relationships between Kira and Lacus and Athrun and Cagalli, that Rau and Mu are genetically related (the two are shown together with DNA strands in the background), and it also shows Cagalli in a soldiers uniform (She has only a cameo in the first episode and doesn't appear again until the opening changes)
The third opening shows Freedom, Justice, and the Druggie Trio's units a good 6 to 10 episodes before they first appear as well as the trio themselves. It also spoils Nichol's death (he and his Gundam are gone from their usual shot) and spoils Athrun's HeelFace Turn by showing him and Kira fighting together. It also hints at Natarle's leaving the crew. Also, the second ending is missing Tolle.
The third opening of Gundam SEED Destiny shows Kira with a new Gundam, spoiling both its debut in episode 39 and that he survives the destruction of his old suit in his duel with Shinn in episode 35. The third opening started airing back with episode 25.
Since the English dub skipped "Ignited" (the first OP) and went straight to "Pride" (the second), plot elements get shown up to thirty episodes early, like the Freedom and the Archangel coming back (episode 13, though Ignited also shows the Freedom and the Archangel), the Destroy (episode 31) and a silhouette of the Destiny Gundam (episode 35).
The second opening shows that Mu La Flaga survived. And the third opening clearly shows his face in a massive close up.
As is the norm for anime aired on Italia Uno, Gundam Wing had its own unique opening for its TV broadcast. Whilst there are some clips taken from the first Japanese opening and some episodes, around three quarters of the footage used for the Italian opening is taken from the OVA movie Endless Waltz, meaning some scenes are spoilered and the new designs for the Gundams used in the movie are designs that are never used in the anime series.
The opening of Monster sets an entirely different tone from what the first episode seems to promise - a feral, disheveled Tenma running with a gun, after you've just seen him peacefully walking around the hospital in his scrubs. Even after things go bad, it takes more than fifty episodes for Tenma to look as worn-down as he does in the theme.
Well, except for the whole 'Shiina turning into a kaiju and destroying Tokyo' part. Which was kind of a pity. That part actually does contain a spoiler from the manga, although the anime ended before Shiina was vaporized when a nuclear missile exploded.
Naruto's openings occasionally have spoilers, although some avert this by changing mid-season in order to update themselves. In general they tend to hint with symbolism, rather than outright spoil the plot.
The third opening spoiled Sasuke learning Chidori.
The fourth averts this, changing parts of the opening prior to its reveal. (Like Kurenai and Asuma's fight with Kisame, Kakashi's fight with Itachi, Naruto's fight with Kabuto, Sasuke's use of Chidori against Itachi, Tsunade and Shizune, Orochimaru and Jiraiya summoning Manda the Snake and Gamabunta the Toad, and Naruto's Rasengan).
The fifth opening (to some degree) spoils Sasuke's defection from Konoha. Though at least it's kind of clever about it like having a shot of an arm reaching to pick up a Leaf headband with pictures of Naruto and Sakura on either side of it, only for the pictures to change to Orochimaru and Itachi after which the hand lets go of it.
The first Shippuden opening heavily hints at Gaara's death, though he gets better by the end of the arc. While this is heavily hinted at, the subtext is only more obvious after the fact, making this a case of hindsight bias.
The second Shippuden opening spoils the mission to capture Sasori's spy resulting in leading Naruto's team to Sasuke himself.
The seventh Shippuden ending heavily hints at Asuma's death.
And in the sixth Shippuden opening, it's made pretty clear that Jiraiya will die very soon. It also hints at the near release of the nine tailed fox and Naruto meeting his father in the mind scape, though the later is also cleverly pulled off by having him shedding a single tear while floating in glowy whiteness. Oh, and the corresponding ending has a shot of the Fourth Hokage in it.
The seventh Shippuden opening showcases that Pain and Konan are invading the Leaf Village a couple episodes before it actually happens as well as shots from Nagato, Yahiko and Konan's past which are from scenes that are not revealed until Naruto's conversation with Pain. It also spoils Naruto's Sage Mode ability.
The ninth Shippuden open spoils Kabuto's return, Naruto meeting Bee (thus implying that Bee survives his fight with Kisame), and Naruto going on a boat trip with Yamato, Aoba, and Might Guy.
The thirteenth Shippuden opening shows the five Kages fighting Madara Uchiha and Naruto and the Nine-Tails fistbumping.
The fourteenth Shippuden opening has Nono giving Kabuto his glasses, along with Edo Tensei's release and having Rin's dead body shown before Tobi who is really Obito and loved Rin.
Ending 28 of Shippuden shows Obito up to Rin's death, then him cradling her lifeless body as he begins to undergo a FaceHeel Turn, as well as a montage of his crimes - contrasted with the song's beginning, which shows him as the ambitious, sweet kid he was before.
Shippuden opening 15 trumps all previous examples. It shows the resurrected Ten-Tails, Naruto cradling Neji's body, Hinata crying, and then Naruto and Hinata Holding Hands, followed by a montage of Kakashi fighting an unmasked Obito, as well as Hashirama fighting Madara, and even Hashirama and Madara as children. Also depicted are the four resurrected Hokages, and Naruto and Sasuke seemingly fighting together - hinting at Sasuke's HeelFace Turn. Taking it Up to Eleven is a shot of Might Guy looking to be undergoing a power up, hinting at a certain subsequent manga development, and the final shot even shows Obito as the Ten-Tails' Jinchuriki.
NEEDLESS: The second opening of the anime very strongly hints that Cruz's sister Aruka is alive and is the last member of Simeon's Elite Four whose identity even Disk didn't know.
In Neo Angelique - Abyss the first opening spoils Rayne belonging to the artifact consortium, J.D being an artifact, Nyx being Erebos and the death of Carlyle. If you had played the game however you would already know these things.
The opening of Neon Genesis Evangelion promptly spoils the fact that there are going to be three main Evas, and shows all the pilots before they are even introduced. There is, however, a nice subversion: the footage is laden with black screens holding text, appearing for a couple of moments each, showing the series' terminology that only starts making sense when it is explained what a particular word or logo stands for. Not to mention the Bait-and-Switch Credits nature of the opening.
By contrast, the ADV trailer imitates the opening of the anime and is practically incomprehensible. It also features one hell of a call ahead as well - Misato having been shot.
The anime's first opening was decently light on this, extensively showing only the hero's True Companions (which he assembles over the course of 30-odd episodes) and their fighting styles. Antagonists (as well as super-spoiler character Dragon) are given half-second flashes.
The second opening covers canon from Loguetown to Little Garden (with a Filler Arc thrown in for good measure), so there are no new crewmates to spoil. Upcoming characters - Apis, Baroque Works, Laboon & Crocus, and the Giants - are again given half-second flashes.
The third opening would be a full aversion... if it weren't for showing Miss All-Sunday, aka Nico Robin as part of the crew, ruining one of the series' biggest twists. The fourth is pretty much a full aversion, unless you count the brief flash to Robin's past (which even the manga hadn't revealed back then; it's possible the anime writers only got vague hints from Oda).
From the fifth opening and on (barring the seventh, as of this writing still the only 100% spoiler-free opening), Toei Animation pretty much threw up their hands and went "Screw it, everyone watching this probably already reads the manga" and started spoiling not just characters but plot-points with impunity. The Davy Back Fight (including Luffy's new look!), Admiral Aokiji's ice powers, the Luffy-Usopp duel, the invasion of Enies Lobby... it got a bit hilarious later on, where the slowed pacing of the anime (combined with the manga starting to favor much longer arcs) meant that the average opening could "only" spoil one or two arcs, where they could once cover a whole saga.
The 22nd opening song OVER THE TOP lives just up to that name by having ridiculous amount of spoilers for those that don't read the manga. It actually opens with a spoiler! To list some, it spoils Luffy being imprisoned after getting one-shotted by Kaido, Sanji's Raid Suit appearance, Hawkins being in this arc in addition to his new One-Winged Angel form of his powers, Orochi and his powers, Yasu's execution, and Kaido's Dragon form going against Gear Fourth Luffy. It's safe to say if you're a follower of the anime and don't read the manga at all, then stay the hell away from this opening at all costs!
And the spoilers given by OVER THE TOP don't end there. That OP managed to do the impossible and spoil things for the manga readers as well such as the implication that Kyoshiro is one of the Nine Red Scabbards and a new Gear Fourth form.
Oreshura initially only has Masuzu and Chiwa competing for Eita's attention; but the opening and ending reveal two other Unwanted Harem-ettes, who don't appear in the actual show for several episodes.
Paranoia Agent creates a deliberate Spoiler Opening to help with foreshadowing. If a character appears in the opening credits, the audience knows to watch them carefully, even if they've only made brief appearances so far. In fact, one particular sequence of the characters has them appear in the same order as they get attacked by Shonen Bat.
For Persona 4: The Animation, just like in the game, the opening gives away many of the kidnapping victims, as well as the identities of the full investigation team. If one hasn't played the game, the script and direction of the anime is clearly intended to make a Red Herring out of Naoto, but simply watching the credits pretty much tells you she's a good guy all along.
The opening of Persona 5: The Animation makes not-so-subtle suggestions about future revelations in the story, chief among them Goro Akechi being the traitor and Caroline and Justine being two halves of a singular entity.
Gainax' Petite Princess Yucie reveals the other princess candidates Yucie will meet up with, who are all prominent in the storyline specifically on their roles.
Pikaia: The opening animations of both seasons roughly summarize the entire season's plot. But the second season's OP in particular shows that Pikaia and Black Pikaia will unite, which doesn't happen until the final episode, and Black Pikaia was a villain until then.
Most of English openings spoil future captures and evolutions that the characters are going to have. The Japanese openings and endings are usually better about this since they change to match the trainers' current parties, but some spoilers still slip through.
The Japanese first opening shows Satoshi's Pidgeot fighting a Fearow. His Pidgeotto doesn't evolve until it fights a Fearow, which happens at the end of the first season, which is also the episode where it leaves Satoshi's team.
The "Advanced Challenging" opening spoils at least 4 evolutions and 2 captures.
Dawn's Swinub evolved into Piloswine during the Sinnoh series, and the Japanese opening changed to accommodate this... except they messed up and used what was supposed to be the next opening, which showed Dawn with a Mamoswine. Sure enough, Piloswine evolved shortly after.
Played with, in the opening credits prominently feature Madoka in her Magical Girl form, making it obvious she will transform fairly early on. She doesn't. As the episodes went on all the main characters except Madoka had made contracts. By the tenth episode we realise that the whole point is that she doesn't become a magical girl, making the opening a from of Bait-and-Switch Credits. However, the trope is eventually Played Straight when Madoka transforms in the final episode.
The song played over the opening credits is also misleading: it is not, despite appearances, from Madoka's point of view. It's from the perspective of Homura during her endless quest to save Madoka from death and/or witchification. This isn't made obvious until the tenth episode.
The opening for the 2 Compilation Movies, however, plays this straight. It has many scenes of Madoka and Homura acting like best friends, or possibly lovers—but Homura is the antagonist, initially, and doesn't seem to care about Madoka very much until halfway through the second movie. A shot of Homura in front of a giant clock surrounded by gears strongly implies she has time-manipulation powers, a big spoiler from near the end of the first movie. It still gives a misleadingly upbeat vibe, though, making it seem out-of-place when it plays right after the extremely dark opening scenes of the second movie.
RahXephon does this very subtly. At first, it seems like nothing but a series of beautiful rapid-fire images. Then you notice the two recurring characters who keep showing up together...
Rainbow: Nisha Rokubou no Shichinin has seven protagonists, all of whom show up in the opening, but only six are seen in the group shot. The group shot shows them in ordinary clothes rather than prison uniforms, and since they didn't meet before prison, they must be gathered after release—without that seventh person.
The opening of Revolutionary Girl Utena spoils the fact that Nanami becames a duelist, which is only revealed at the end of the first season. It also shows you the face of Utena's "prince" waaaay before he actually appears.
The Italian dub lyrics for the first season's opening states outright that Sailor Moon is the reincarnation of Princess Serenity; the third season's opening showed that two of the Talismans were in the hearts of Sailor Uranus and Sailor Neptune.
The DiC English dub opening, which never changed anyway, does completely kill the mystery of who the other four Sailors are by showing and naming them right in the theme song.
In the original anime, the third opening of the first season pretty clearly shows Sailor Moon transforming into a pretty and royal looking dress which makes it obvious she's the Moon Princess. It started being shown shortly after Sailor Jupiter showed up, 9 episodes before Princess Serenity first appears. Otherwise, Sailor Moon was usually pretty good about this, especially during S when they used multiple variations on the same opening to hide the new Senshi, the true villains, Super Sailor Moon, and other elements. Which was then completely blown in the English release because Geneon was only able to get Toei Animation to give them the final variation of the opening, which of course spoils the entire series. They were eventually able to coax the original variations out of them as extras, but the damage was already done.
The first season openings concealed some of the Senshi's identities, but even the very first opening showed Sailor Mercury and Sailor Mars along with Sailor Moon. The two weren't introduced until episodes 8 and 10, respectively.
Sailor Moon does spoil the true identities of Tuxedo Mask and the Moonlight Knight in the credits.
The opening of the third season, Sailor Moon S, quite blatantly shows Sailor Neptune and Sailor Uranus as a part of the team, which was slightly jarring, given that Neptune and Uranus had a rather antagonistic relationship with the inner senshi. Only by the end of the season they truly acknowledged Sailor Moon as their friend and leader.
Not as coy as the original's Evolving Credits, Sailor Moon Crystal'sTitle Sequence shows all of Usagi/Sailor Moon's superheroic supporting cast both in-costume and out, as well as smiling versions of Queen Beryl, her generals and Big Bad Metaria. Queen Serenity is also briefly shown at her Moon Castle.
The second opening of Crystal gives away Haruka's true gender, as well as Pluto's return and the true identities of Sailor Uranus, Neptune and Saturn.
During the Asgard Saga opening, you can see who gets to wear the Odin Robe.
Hades Inferno where they show Shun as Hades.
Watch the S Cryed OP again. Pay attention to the brief hilights over Kazuma's body. It shows the exact order of his upgrades. Plus completely spoils the Ryuho's change to his Alter by doing the same type of light-up sequence.
Seraph of the End showed Mika (with fangs no less) in the opening in episode 2. In the previous episode he had "died" and it hadn't been revealed he was alive yet in the anime itself.
Sgt. Frog. There's a fifth member of the Keroro Platoon! They keep talking about him vaguely and using silhouettes and.... oh, wait, who's the light blue guy in the theme song?
The second opening to Shaman King shows that one of the scariest bad guys from the first season will end up on the side of the good guys. In fact, the first opening does this for two others as well.
Shiki does this with it's first OP. By showing who's going to be turned into a Shiki by skullifing them.
The first op for Shin Mazinger is a subversion in that most of the stuff in it is footage from the very first episode.
However the second just cranks the spoilers Up to Eleven. In 85 seconds, we learn that some bad guy (most likely a Mikene, a member of Zeus' race) comes to earth, and boy does he look pissed; we get a good look at some of the enemy robot designs that'll probably be used in upcoming episodes; we also learn Mazinger is going to fight an Evil Twin; ever wondered how Mazinger turns into the Big Bang Punch? Well its right there for you! Baron Ashura seems to be about to relive past experiences; Tsubasa is going to have a really bad day; and we get a nice good look at Tetsuya and his mecha, though it might not be Great, since it really doesn't resemble it all that much.
Shion no Ou: The mysterious killer of the protagonist's parents' face is strategically covered whenever it gets close. This can be pretty tension-filled for the watchers...if it weren't for the fact that it's fully shown in the opening.
Simoun: The major plot arc of the first eight episodes is Aaeru's attempt to get Neviril to pair with her, and Neviril's refusal to fly with anyone during that time. However, the opening credits prominently feature the two flying together, promptly removing any and all suspense that they might not become a pair, and show a number of other characters who have not been introduced yet.
The opening of Sky Girls spoils the appearance of Elise quite a while before she even shows up and only several episodes after that she actually joins the cast proper; and it basicly spoils the entire rest of the cast too. It also shows a girl whose face is obscured by a helmet; it's actually Aisha who joins very late and otherwise only showed up once to fight against Eika in a test battle.
Slayers tries to set you up to see Rezo as a good guy and Zelgadis as a villain. Now look at who Lina and Gourry are attacking in the opening and who shows up in the "hero" section of the Team Shot at the intro's end.
If you know what to look for, the Slayers Next opening spoils Gourry being kidnapped, the appearance of Lord of the Nightmares (twice, even!) and the Ragna Blade. Ironically, it doesn't spoil the anime per se, but the original novels, by showing the Sword of Light being lost to the darkness.
The sigil for Dark Star Dugradigdu briefly flashes at the opening of the third series Try, descending towards the planet. Yes, this is important. (The blink and you'll miss it graph afterwards is known as the Alternate Plane Chart.)
Less noticable, but a quick eye makes it quite obvious that Xellos is a Mazoku inside the Slayers Next opening.
Slayers Evolution-R is the biggest offender for being an original story and yet still having an opening that spoiled the climax. Pokota's sealed body is shown opening his eyes with the symbol of Shabranigdu in them, and then it cuts immediately to Lina fighting Shabranigdu and then a scene of her casting the Giga Slave. Guess what happens! Many fans thought it was so obvious that it had to be a Red Herring.
The dub opening for Sonic X shows Shadow opening his eyes. Shadow doesn't appear until a little while into the second season. He's also seen in the Japanese opening. To be fair, much of the fanbase was made up of people who beat some of the games. They had whole arcs spoiled for them before the series premiered.
The second opening shows Stein's utter loss of sanity well before it actually happens.
Chrona makes a prominent appearance in the first opening, so we end up watching him/her for several episodes before he/she actually appears.
The fourth ED is essentially a spoiler for the end of the series; a monstrous Asura being fought by the main teams, the adults rendered useless (if not apparently dead) with the kids left to save the day and Soul and Maka featuring most prominently. One point that doesn't correlate to the finale is that it's Soul supporting Maka - similar to the switch around in the second OP - when again it's the other way around. Another is that a panicked Crona is standing over what appears to be a small child's corpse, implying that zhe killed Medusa even though she was still possessing Rachael—but Crona actually couldn't go through with it, then Maka managed to defeat Medusa without harming her hostage.
Spider Riders: The second OP is really bad about this. For one, it shows Aqune with the Spider Riders, even though she doesn't officially join them until the fourth-to-last episode. Next, it reveals the existence of Quake/Brade, as well as the fact that he's a Spider Rider, and has a younger form. Although viewers could assume that younger Quake/Brade was a different person entirely.
The fact that Perrine's mother Marie only appears in the first part of The Story of Perrine's OP sequence implies that she dies midway through the story. Then again the anime is based on a book that begins with Marie's death, so...
Other scenes show Palikare finding and nudging a passed-out Perrine (which happens some time later) and Perrine living in a hut (in the Maraucurt days)
Both used and subverted in Str.A.In.: Strategic Armored Infantry. The initially important Isabella doesn't show up in the OP, and she gets killed off. This led a lot of fans to suspect that the next victim would be anyone but Sara, Ralph, Lavinia, Lottie or Carris, as those five appeared in the OP. Carris was the next victim, and many not included in the OP survived.
The series also features a spoiler ending which depicts the final scene of the series.
The opening credits for the second season of Strike Witches prominently features the battleship Yamato in several shots, and true to form, she plays a significant role in the final battle.
The second opening of the anime Sword Art Online pretty much spoils what happened to Asuna at the end of the game, where she is, the fact that Yui is there too, and what is going to happen from episode 16 onwards. The first opening isn't really an example of this trope, as the biggest spoiler it has is Asuna's face behind the cape she wears in the first part of episode 2.
Considering all it spoils, the second opening is surprisingly good at not revealing that Lyfa is Suguha's avatar. The second ending however, strongly implies that Suguha will play an important role in the story, by making her the sequence's main subject.
The opening also subtly spoils the fact that Death Gun is actually three people, as shown by the three shadows of Death Gun harassing Kirito. It also not-so-subtly spoils the fact that Shinkawa is one of those Death Guns by burning up his scene to reveal Death Gun in a manner reminiscent of a Transformation Sequence.
The second version of the second opening of the second season starts off by showing two girls who quickly grow up, until only one is left, at which point she's shown to be Yuuki, the new character in the Mother's Rosary arc. This spoils the existence of Yuuki's twin sister.
The first opening of Alicization shows an Integrity Knight who looks exactly like Eugeo's long-lost Childhood Friend Alice, which spoils that Alice becomes an Integrity Knight.
Subverted in Tenchi Muyo! GXP. During the opening, four silhouettes appear during the initial opening, as well as a small shot of the Earth near the end. As episodes progress, the silhouettes are revealed to be one of the main cast as well as OVA mainstays Ryoko, Ayeka and Tenchi. The shot of the Earth later adds another battleship as well as spaceship mode Ryo-Ohki as they appear.
Played straight in both Tenchi Muyo! and Tenchi Universe as the first opening of the OVA shows off Ayeka, Sasami, Prince Yosho and Ryo-Ohki ship form before their first appearance in episodes 2 and 3 while Universe's opening shows off all six girls despite taking five episodes to get them all around.
The first opening for the show, even though it's shadowed, features the combination between Gurren and Lagann.
Averted later on in the series. Kamina features prominently in the intro until the episode after he dies. Also, the full Gurren Brigade and Dai-Gurren are absent, despite their importance, until this point. There's also an interesting subversion at work here considering Thymilph doesn't appear in the opening until AFTER he dies.
Although the opening for the post-Time Skip section does somewhat spoil the ongoing bit where Gurren Lagann starts combining with every battleship the heroes get, transforming into forms each ridiculously huger and more powerful than the last if you have good eyes when it shows Lagann and Gurren combining. It also spoils Nia's status as a Manchurian Agent.
The Law of Ueki spoils characters and scenes aplenty in both of its opening sequences, using actual footage from the show.
Tokyo Mew Mew shows off all 5 of the Mew Mews in the opening credits, which stayed the same throughout the entire series. Not a huge spoiler mind you, but the way the girls react make it seem like a big surprise. Less of an issue in the manga since Ichigo sees all of the other girls at the zoo when they're infused with the animal DNA.
Toradora!: Ami Kawashima appears in the opening from the first episode on, even though she appears first in the fifth episode.
The first opening of Toward the Terra shows Shiroe getting killed by Keith. This is done so ambiguously though that the viewers won't really get it until they get to that part of episode 9. The second OP also shows Tony and the other Nazca children rapidly growing up. Though with the series love for Time Skips, people might not take that part of the opening as literal as it is.
The lyrics to the first opening of ∀ Gundam not only tell the viewer that the Black History was intentionally forgotten, it also tells them why. When Agrippa explains this (40 episodes into the series!) he almost quotes the lyrics.
Beatrice's portrait changes each arc. The second arc shows Beatrice in human form and a portrait of Eva-Beatrice, and the third shows Eva-Beatrice, and changes the rest of the animations to include the Siestas, Virgilia, Bernkastel, and Lambdadelta.
The lyrics are also basically a summary of the most of the main plot, and give strong hints of what's coming.
Utawarerumono's opening goes through visuals of nearly all the major good guys, allowing you to predict nearly every single HeelFace Turn in the series. What it doesn't spoil is the bizarre ending.
The anime version of Valkyria Chronicles. The second intro involves Alicia activating what looks like Valkyria powers to fight against (proving that she's a Valkyria) Selvaria who just does the same thing, since she the one that introduces what the hell kind of super powers a person with such a bloodline has.
The opening for Vampire Knight shows Yuki, a human character, drinking blood. The ending shows her with glowing red eyes and bloody tears. It doesn't come as a huge shock when she learns she's a true vampire.
The The Vision of Escaflowne opening spoils Escaflowne's Dragon form, whose first appearance is treated as a cliffhanger surprise at the end of Episode 4. It also has a shot of Escaflowne fighting Sherazade on a desolate battlefield, a fight that happens near the very end of the series. And the winged, angel-like Draconians in their city.
Your Name has in its opening some things like a certain writing on Mitsuha's hand or showing the two leads as adults, things that won't happen until late in the film.
The 4Kids opening of the first season shows a clip or two featuring the Magician of Black Chaos. The Magician does not actually make an appearance in the anime until Yugi's climactic battle with Pegasus. As a whole, the opening often features shots of monsters that will appear that season, and if you can identify the monsters seen you can often take a pretty good guess at which duelist will be using said monster.
The second Japanese opening, "Shuffle", manages to spoil the fact that Yugi obtains Slifer the Sky Dragon/Osiris. If you look closely at the scene near the end of the opening (which features Yugi, Marik and Kaiba), you can see the Slifer card in Yugi's hand.
The opening itself came in at the end of episode 49 (weirdly the last part of the Dungeon Dice Monsters arc, not the start of Battle City), and Yugi got Slifer in episode 67. To further make it a slight spoiler, Kaiba is shown twice with the God of Obelisk/Obelisk the Tormentor card. He obtains the card in episode 52, just 3 episodes after the opening came into use, so it's not as major a spoiler as Yugi getting his God card. The only other major spoiler for this opening is Yugi summoning the Black/Dark Magician Girl, a monster that doesn't appear until 12 episodes later.
In the "Enter the Shadow Realm" sub-arc, the arc shows Gozaburo Kaiba confronting the heroes when he doesn't show up in the arc until the very end and is supposed to be dead. A more subtle spoiler shows Noah, the main antagonist of the arc, in Mokuba Kaiba's clothing - Noah is later revealed to be the third Kaiba brother.
The Yu-Gi-Oh dub's first season opening shows a lot of scenes that don't happen until well past the Duelist Kingdom arc, particularly those of the first time the crew entered the Virtual World and thwarted the Big Bad Five of KaibaCorp.
A spoiler for the Italian audience - the opening for Yu-Gi-Oh! 5D's in Italy is basically the Hyper Drive sequence, with 5 seconds added from the second version of the sequence to fit the song lenght. Naturally the 5 seconds they added to extend the sequence featured Yusei facing Kiryu/Kalin and Crow and Blackwing Armor Master. Keep in mind these characters won't be seen for around over 30 episodes, and whilst the US intro that came in from episode 27 showed this, the Italian version showed the two characters from the off.
Even the Japanese version of 5Ds began doing this, with the forms of Shooting Star Dragon and Machine Emperor Grannel. Initially, for the fourth opening, they were obscured (the former in light, the latter in shadow). This had been done before, with the creature being added the episode after they appeared. With these two, however, they were added into the intro of the very episode they debuted in, thus ruining the anticipation.
The second ending to Yu-Gi-Oh! ARC-V shows Sora mysteriously absent from his group of friends, spoiling that he's not the cute kid he seems to be before it's actually revealed. It also shows him very angry and disheveled after showing an angry Kurosaki, showing the rivalry the two would have in their match. Finally, it also spoils Yuto and Yuya fusing together; Yuya would later "absorb" Yuto into him after the latter loses to Yugo.
First opening spoils (at least) the first 14 episodes... Yusuke coming back to life, Kuwabara gaining powers, Kurama and Hiei's surviving AND their HeelFace Turn.
The first ending spoils Koenma's human world form, as well as showing Yusuke fighting Chu and a shot of Team Toguro long before their debut. The third ending spoils Genkai's revival and that not long after Yusuke's team wins the Dark Tournament, Sensui replaces Toguro as the Big Bad (It debuts just before the final fight with Toguro).
The first few episodes of Ergo Proxy make it appear that Re-L is the protagonist of the show, when in actuality the true protagonist is Vincent who in the first few episodes is a meek loser of a suspect, with his Eyes Always Shut and his hair smoothed flat. However, in OP, he has more prominence than she does, and his eyes are open and his hair wilder. Perhaps due to this, the first 2 episodes have no opening, with the credits playing as the episode begins.