Legend of the Galactic Heroes. It's heavy with politics, 110 episodes long and if you miss just one episode, you might end up not understanding what's going on anymore or how things came to be. Add three movies and two 24 episode sets of "Gaiden" episodes to the mix and you have quite the ride in front of you. Slightly alleviated by the fact that the episodes are around the 30 minute standard for anime rather than the 1 hour or longer OVA usually made in the more recent days.
Gundam: This happened with the original UC timeline, which is one of the main reasons Alternate Universe series were made. It was also a major driving force behind the creation of Metal Armor Dragonar; Bandai wanted to bring in fans who might have otherwise been stymied by the existing Gundam mythos and were ready to switch production to Dragonar if it outperformed Gundam ZZ. It didn't, but remains a cult favorite. Somewhat remedied by the easy-to-follow-if-hard-to-grab-all-the-nuances Mobile Suit Gundam Unicorn.
Parodied in an episode of Sayonara, Zetsubou-Sensei: Itoshiki was driven to despair by, among other things, the fact that his own show had so many running gags that it was impossible for new viewers to understand. Hence, he changed the screen so that it displayed constantly changing information about all the characters and their personalities, and went on to explain several of the nominal puns and running jokes. It was also parodied in that they quickly grew tired of those longtime viewers who knew the show so well they saw all the jokes coming.
This is cited as a major problem with Digimon: The Movie. Unless you're already familiar with the franchise, the overall plot will be all but incomprehensible, and the plotholes induced by the dubs edits, resulting from decisions such as cutting down the Hurricane Touchdown segment to a third of its original running time and shoehorning it in with the other two segments of the film (albiet that was at executive insistence), despite barely relating to their events at all and having a whole new set of characters dropped on the audience, aren't helping.
Death Note, mostly due to the Gambit Pileup nature of the series. It's possible to jump in within the first ten episodes or so, but after Light and L actually meet each other, forget that.
Given that One Piece has been running since 1997, Eiichiro Oda understandably tries to avoid locking out his readers, which can be difficult given the fact that almost everything and everyone in the series is of some importance even if you don't follow the series from the start. Given that collecting every volume of the story released so far will set you back a few hundred dollars, he understandably puts short flash-backs into the story as well as summaries of the various arcs. To offset this, volume 50 is clearly labelled as a good "starting point", complete with recaps, backgrounds and a new direction for the story.
The OVAs of both Tsubasa and xxxHolic went straightforward with several arcs, skipping out several important details that would make anime-only viewers very confused. In Tsubasa Shunraiki, it adapted the Nihon arc but skipped the Infinity and Celes arcs, meaning anime-only viewers missed out Fai's backstory. In xxxHolic: Rou, it starts with Yuko dying and Watanuki inheriting the shop. However, the circumstances behind it were vaguely explained in the anime and everything tied to Yuko and Watanuki's backstories were in the Tsubasa manga, particularly the Clow arc which was never adapted.
Pokémon Adventures has its instances of this, Each new arc in the series (called "Chapters") features a different protagonist in a new region going on a different adventure from the previous protagonists. However, sometimes the previous protagonists will make cameo appearances and interact with the new protagonists. A new reader might be confused when picking up one of the more continuity-heavy Chapters, like the Emerald Chapter, which has a Bat Family Crossover in the grand finale featuring every single main character from the past 3 generations. Even worse are the remake chapters note such as Firered & Leafgreen; Heartgold & Soulsilver; and Omega Ruby & Alpha Sapphire, which act as sequels to the original Chapters note Red, Green, & Blue; Yellow; Gold, Silver, & Crystal; Ruby & Sapphire; and Emerald. If you haven't read the original Chapters, you definitely won't be able to understand what's even going on in the sequels.
Just about all the Pokémon: The Series movies are completely separate from the series and themselves, so one needn't see them to understand something in the series. That said, they assume you're already familiar with the series itself, as they make little effort to establish who the main cast are or what they're even doing. The first few movies, particularly Pokémon: The First Movie, are even worse — that one expects you to know what Pokémon are, who Giovanni is, why he's so important, and many other aspects someone unfamiliar with the franchise would be totally lost at.
There's also the infamous case of Banned Episode in the series, which some banned episodes can throw viewers off. One example is the controversial episode which features depictions of firearms. In that episode, Ash catches 30 Tauros and sends them to Professor Oak. The Tauros appeared later in the series even after the episode did not air outside of Japan.
Code Geass: So many things happen in each episode, especially in the super-fast pace of R2, just skipping one episode would result in no understanding of the current plot. And it's difficult to understand the plot even while watching normally.
Code Geass: Lelouch of the Re;surrection is a continuation of the compilation movies that changed several plot points. So, if one were to watch this movie without seeing the compilation movies, they would be confused about certain details such as Shirley being alive despite being killed by Rolo in the TV series and C.C. wanting to bring back Lelouch despite moving on from his death.
The first season isn't so bad. Its episodes are cleanly split between "Stand Alone" and "Complex" episodes. Stand Alone episodes are entirely self-contained storylines that are wrapped up by the end of each episode. They're not filler episodes (as they reveal background info for the characters, among other things) but they're not related to the main plotline that gets told in the Complex episodes.
The 2nd season gets a little more complicated. Episodes are divided into "Dividual", "Individual", and "Dual" episodes. Dividual episodes seem to be like Stand Alone episodes, except all of them have some minor detail that becomes more important later on. All three episode types cover three different angles to an overarching storyline that eventually unfolds.
Following along with the story can be quite confusing if you don't watch either season from the very start, or even if you end up missing even one episode, as the series doesn't use Previously on…, and don't use Recap Episodes in the same obvious way that most series do.
And all of this headache isn't even including how the original manga and the Mamoru Oshii films are by themselves separate continuities that have nothing in common with each other. Anyone who's just starting into the series will inevitably ask what the correlation is between the manga, the movies, and the anime series.
This also proved to be one of the many problems with the 2017 live-action remake. The story ended up confusing fans with the changes made to the source material and also gave more casual audiences questions about references to the original story they didn't understand.
Anybody introduced to the .hack series as an anime can potentially run into a giant wall of Lock-Out. The two main anime series, Sign and Roots, are actually prequels to the main stories of their respective Myth Arc which are told in two sets of PS2 games, and thus (although the former does resolve its main story) don't resolve many of the major story-arc significant plot-points of the respective eras. Made worse with the second anime "Legend of the Twilight Bracelet" being the non-canon version of its respective manga, and Roots having been rendered non-canon, with most of its depicted story (though not its myth-arc) events suffering from Canon Discontinuity to the point it's recommended to just skip those two entries entirely. The effect is even worse if you read the novels or manga, which are generally side stories (or for the novelizations/adaptations of the video games Alternate Continuity), with huge references to the main plotlines that aren't well-explained in that form. The games leave some things unexplained (the system restart during the climax of Quarantine comes off as an enormous Deus ex Machina... unless you watch Liminality, which explains why that happened), but left ambiguous enough that that section of the story such doesn't seem necessary. And that's not even getting into .Hack//Link which has its story tied to revisiting the stories of the past parts of the franchise, only for them to get many if not all of the details wrong. While the franchise DOES explain some events in brief that are expanded more on in other material so as to not leave a newer inductee completely lost (you can pretty much just pick up each of the two PS2 game series stories and still get a complete story out of it), you may not fully appreciate this series unless you're prepared to read/watch/play all of it.
In Demon Slayer: Kimetsu no Yaiba, following the Kimetsu Gakuen/Slayer Academy spin-off is a daunting task in its original published format, since its contents are produced in a very spread-out capacity, different magazines, different publications between manga, light novel and even databooks. The anime makes it easier to consume, by being post-episode extras, until the Valentine arc that was originally a stream-only event. To know the deeper details of many characters and their motivations is a hard task for people in places where both Fanbooks (Databooks) where not made officially available, and as far the anime is concerned the staff is treating the Fanbooks as negligible in adapting to scenes for the animation. To top it all off there’s the Koyoharu Gotouge Exibition, an art exhibition which took place in Japan, 2021, where the series author revealed a little more new details about the series’ epilogue through art pieces and articles, if one didn’t physically attend and took some pictures to keep it, then such new info is completely out of reach for everyone else.
Three words: Hajime no Ippo. The series has been running for decades and has already clocked in at over 1,000 chapters with no real end in sight. Sure, you COULD jump in later since it's about boxing and thus all you'd need to know is "this character won/lost some previous matches," but without knowledge of who certain characters are or how/why they box a certain way would make it difficult to follow.
Vision of Escaflowne is guilty of this, mostly due to focusing ONLY on its story and characters. It's carefully plotted and compressed, the content in the episodes always leads up to something, and there's absolutely NO filler in it (this would have been rectified had the producers had the necessary budget and amount of episodes they wanted), not that this prevented fans from loving it to death.
The third Rebuild of Evangelion movie is next to impossible to follow if you haven't seen the original series. For one thing, the vast majority of the Continuity Nods and Mythology Gags make no sense otherwise, and the revelation that Kaworu is the final Angel comes out of nowhere.
JoJo's Bizarre Adventure: Although the manga is divided into Parts that tell mostly self-contained stories, smaller details will only make sense to a reader who followed the overarching plot from the beginning. Parts 1, 2, 3 and 6 are strongly connected to each other; though the other Parts tie into the main story in slightly less important ways.
Opposite to the Dragon Ball anime, Dragon Ball GT is almost incomprehensible without prior knowledge of DBZ's storyline, particularly since it didn't refer back to DBZ that often like DBZ did to Dragon Ball.
Dragon Ball Super has a minor case of this since the show starts on the assumption that the viewer is aware of the events of the Buu Saga. This has caused much confusion for many newer English fans who grew up watching Dragon Ball Z Kai which only went up to the Cell Games and Buu Kai didn't get an English premiere until Super was airing at the same time on Toonami. Also, since the show runs on nostalgia, all the Continuity Nods and Call-Backs can be hard to follow, especially the Future Trunks Saga that mixed important plot points from both the Cell and Buu Sagas. The show also assumes the viewer read or at least have passing knowledge of Jaco the Galactic Patrolman, since it doesn't bother to explain where Jaco came from, how he knows the Briefs, when did Bulma get a sister, or who are the Galactic Patrol. Toei attempted to avert some of this by retelling the movies written by Toriyama, so people didn't have to go out and buy the DVDs to catch up with the latest events. You can even skipped the retellings and jumped to Episode 28, the start of the Champa Saga, since Episode 30 is a Recap Episode of everything that has happened up to the point. And though the Resurrection 'F' Saga draws a lot of parallels and plot points from the Namek/Frieza Saga, it's easy enough to follow since it is a basic revenge story.
HuGtto! Pretty Curereally suffers from this trope. Being a 15th Anniversary season, Hugtto makes many call backs to older seasons that can fly over a newcomer's head at first-viewing, and to make it even worse, they had straight up crossovers with the Max Heart, Maho, and Kira Kira teams, as well as Cure Dream and Cure Peach. The show assumes that viewers has seen these seasons already and thus never bothers to introduce these characters for the sake of moving the plot forward, which can make these guest characters come off a bit on the bland side. It then goes full throttle with the crossovers and has an episode where EVERY PREVIOUS CURE crosses over.
The Hyperdimension Neptunia anime seems to have been made with the assumption that the viewer has already played the games, and thus does little in terms of world building or establishing characters and their relationships. It's not impossible to follow along without having played the games, but important plot points, such as the alternate dimension Plutia and Peashy are from, as well as Rei's backstory and motivation, recieve less of an explanation than one might expect.
King of Prism is a spin-off of Pretty Rhythm: Rainbow Live that focuses on the male characters, and watching Rainbow Live is pretty much a given if you want to understand the lore. While the first movie, King of Prism by Pretty Rhythm, explains Over the Rainbow's story, the second movie, King of Prism: Pride the Hero, does not play around and immediately throws in things that reference Hijiri, Jin, and June's backstories without any primer. Even worse, you have to watch Rainbow Live to be able to understand one of the biggest spoilers in the series, as it provides context. For Western viewers, this is a lot worse, since Rainbow Live was not licensed for Western distribution, and the King of Prism movies only screened at the Los Angeles Film Festival; Avex themselves have made the series difficult to obtain legally since their export ban on home releases.
The Yo-kai Watch movies have this issue. They tend to heavily involve King Enma. Enma, however, isn't a character in the main anime, so unless you've seen previous films it can be confusing just who he is and what's going on.
Kaguya-sama: Love Is War has this by way of Continuity Creep, to the point that people who are coming into the manga after seeing the anime are often told to at the very least go back and read all the chapters that were skipped in the adaptation so they don't miss anything that could potentially become important later on. This is taken even further with the Spin-OffYonkomaWe Want to Talk About Kaguya, whose very premise as an Innocent Bystander Series and brand of humor pretty much requires the reader to be familiar with specific chapters of the main series.
My Hero Academia: The Paranormal Liberation War starts off with a massive revelation regarding Oboro Shirakumo, Aizawa and Present Mic's high school friend. The problem is that Oboro's only appearance in the manga proper are brief cameos that never even give him a name, the character's relevance and backstory, and why that revelation is important to the characters involved, is only apparent if the reader was also accompanying the Spin-Off and prequelMy Hero Academia: Vigilantes, which has an arc dedicated to Aizawa's high school life and his origins as a hero.
Tenchi Muyo! Ryo-Ohki didn't start out too badly, with simply the 2 OAV series, and everything else being its own alternate universe. But then the 3rd OAV came around, which introduced many characters that would be confusing to audiences who hadn't seen Tenchi Muyo! GXP (and even THAT series had a major plot point that only makes sense if you've seen Dual! Parallel Trouble Adventure). Then, the 4th OAV dedicated much of its screen time to setting up Tenchi Muyo: War on Geminar. That not only continued in the 5th OAV, but they also introduced much of the cast from Paradise War, a set of light novels that have not been translated out West. Even barring that, there are a number of characters introduced in the 4th and 5th OAVs who are only properly introduced in the light novels and doujins created between each release. For anyone who is watching the anime only, anything past OAV 2 is beyond confusing.
The Legend of Heroes: Trails of Cold Steel - Northern War might as well be called "Continuity Lockout: The Anime." The anime assumes that the viewers have played through the Trails Series video games starting from The Legend of Heroes: Trails in the Sky First Chapter, Second Chapter, and The 3rd (and one of the events of The 3rd alludes to North Ambria, the nation where the anime takes place, and the Salt Pale Incident), then have played through The Legend of Heroes: Trails from Zero and Trails to Azure as the annexation of the Crossbell State is mentioned a couple of times by the cast, and finally have played through The Legend of Heroes: Trails of Cold Steel I and II as the anime takes place after Cold Steel II but before the events of Cold Steel III as the protagonist of the Cold Steel saga is essentially The Dreaded to the North Ambrian cast. The anime does briefly explain a little bit of the events that had happened before but without prior knowledge of what those events entailed, a first time viewer would just get lost. In short, a viewer would have to at least play through seven games with each game clocking in at almost 70-100 hoursper game just to know what the context of the anime is.
