When Urusei Yatsura was shown in the late 1990s, people complained that it was "just a rip off of Tenchi Muyo!!". Urusei Yatsura had been parodying Tenchi's genre since the late seventies.
Disgusted fans in various threads on several different anime forums complained that Mobile Fighter G Gundam (made in 1994) ripped off the plot and major themes of Zoids: New Century Zero (made in 2001). Their apparent reasoning was that because Zoids was the first show to air on Cartoon Network, it must have also been the first show to have been made in Japan.
Similarly, Mazinger Z often faltered in international releases due to people accusing it of ripping off shows it inspired, such as Voltron and it's own spinoff/sequel UFO Robo Grendizer.
Many people seem to think anime/manga invented the Mecha idea when actually Jules Verne and H.G. Wells had everyone beat by more than 50 years with a steam powered mechanical elephant and Martian Tripod Terror.
Many people seem to complain that the video game series Tengai Makyou (AKA Far East of Eden) is a ripoff in many ways of Naruto, its main offenses being that it's overly Japanese, it features a character that can summon giant frogs, and has characters named Orochimaru, Jiriya (Ziria) and Tsunade. Tengai Makyou first came out 1989, 10 years before Naruto, and parodies much of the same Japanese mythology that Naruto draws on.
Any series that features ninjitsu as a main theme will be accused of ripping off Naruto; for example, Ranma ½ (manga 1987, anime 1989).
Likewise, many elements and concepts that people assume originated in Gurren Lagann were actually derived from the Getter Robo series — particularly ludicrously oversized mecha, the idea of Spiral Energy and the risk of overusing it. And drills.
Eva had spiral energy - background material gives the source of the S2 organ's power as the double helix of DNA(which may just be the reason why TTGL has its universe centered around it).
Speaking of Neon Genesis Evangelion, while it is commonly viewed as Deconstruction of old Idealistic Super Robots show, some of the more notable ones actually have been done before. Most notably, the idea that giving a teenager the power to destroy the world will have bad consequences was used in Mazinger Z, right during the second chapter of the original manga, making it much of an Unbuilt Trope. Also, Tetsuya Tsurugi from Great Mazinger deconstructed the arrogant, HotbloodedAce Pilot archetype long before Asuka Langley Sohryu. And Zambot3 deconstructed the whole genre including the Kid Hero trope - in the late seventies.
People accuse Alucard's design of being a ripoff of Dante's, thanks to the fact that they both dual-wield black-and-white guns and wear bright red dusters. Devil May Cry: 2001. Hellsing: 1997. Dante's design is in fact a tribute to the main character of Space Adventure Cobra (1978).
It's funny people should accuse DMC for ripping off Alucard when at first he elicited cries of "Vash rip-off!" To get even crazier: Vash is a guy who dresses in red, has blonde hair, a huge revolver and a gun hidden in his arm while Cobra is... a guy who dresses in red, has blonde hair, a huge revolver and a gun hidden in his arm.
When NEEDLESS began, many people complained about it shamelessly ripping off of Gurren Lagann due to its animation style and characters (most notably, Blade, who resembled Kamina). What they didn't know was that the manga of NEEDLESS started in 2004, 3 years before Gurren Lagann.
Besides, the notion of a wasteland populated by super-powered people was already explored in s-CRY-ed.
There are Twilight fangirls who seriously think Vampire Knight is a Twilight rip-off. It's hard to say, really, because both of their source materials were published in 2005. but it's safe to say mangaka Matsuri Hino began working on Vampire Knight before the Twilight book was actually published, and translated and reprinted in Japan. Although this is still debatable, Twilight fangirls took it a little too far when they targeted Karin (because, you know, Karin has vampires that can walk under sunlight!).
Many so-called Dragon Ball "purists" insists on not using the term "Saiyan", the anglicized form of Saiyajin, since it's a purely American invention according to them. What those people don't know is that the anglicization of "Saiyajin" to "Saiyan" was first used by Bandai for their Dragon Ball Z toys, such as the "Super Battle Collection" action figures, which predated any of the American adaptations.
It's not even an anglicazation. It's a translation, since the "jin" ending denotes being from that area, in this case "saiya", so the English way of saying that someone is from "saiya" would be "Saiyan."
Berserk was virtually unknown in the US until the 1997 anime fansubs gained a cult following. Because of this, it and the licensed game were often described as taking elements from otherworks with giant swords. It was actually their inspiration.
It doesn't take much effort to find people who accuse Code Geass of being a rip-off of Death Note. However, Geass was in development for five years before it came out in 2006, while Death Note first appeared in 2004.
That doesn't necessarily mean that it's not a rip-off. Anyone who's ever worked on a television show of any kind will tell you that the concept evolves in development, and that new ideas can be incorporated right before or during principal production. Creators aren't known to always tell the truth about their works. Sometimes, the Word Of God is a lie. However, similar ideas executed in different ways? Not a rip-off by a long shot, or everything is a rip-off of something (except Highlander).
Code Geass and Death Note don't really have that much in common besides some superficial similarities: both involve Xanatos Speed Chess and brilliant young men with self-righteous ideals who gain some sort of supernatural power by chance. That's it.
Osamu Tezuka's Kimba the White Lion series has received some flack for bearing similarities to Disney's The Lion King... even though the original manga was created 30 years ago. Tezuka was dead for almost 5 years by the time The Lion King came out! Sensibly, more people make the opposite accusation in this case.
This was parodied in The Simpsons: "You must avenge me, Kimba. I mean, Simba."
As he was writing Astro Boy and Asimov's stories started getting published apparently a bunch of people accused Tezuka of ripping of Asimov outright. Surprisingly enough Tezuka did come up with the concept of laws that had to be obeyed by robots before Asimov (or at least published such a thing first). Chances are that him and Asimov having the same idea was more than likely a coincidence.
The idea for Asimov's laws were actually suggested by his editor. He noticed that in Asimov's work, his robots seemed to follow a set of rules, so he pointed it out to Asimov, who decided to make it blatant text. When thinking about it, the idea that robots need a set of rules to abide by is just logical.
Among Defense of the Ancients players, knowledge of “Lina Inverse, The Slayer”'s origins is sparse enough where fanart (her appearance is a reskin of WC3's sorceress unit) and fanfic has been created with no connection whatsoever to the original character.
Similarly, people seem to think that the nickname "the white devil" is simply one created for Nanoha, whereas it is in fact a a description of the original RX78-2 Gundam from in-universe, applied to Nanoha for ironic purposes.
The ''"Flat chest is a status symbol!" quote is always associated with Konata, as opposed to its original user Mayumi. Which is weird, since Konata flat down says when she uses it she got it from "an eroge character". Admitedly Mayumi doesn't use it on the anime, but still.
When Space Battleship Yamato first aired on US tv, it was sometimes derided as a cartoon rip-off of Star Wars. However, Yamato (pilot episode premiered in 1974) came out 3 years before Star Wars (first theater screening in 1977). Productions came even sooner than that.
Multi-route adaptation of a Dating Sim: If somebody thought Amagami SS was the first to do this, they must not have read Kimikiss - Various Heroines. Developer Enterbrain must has preferred both of their franchises being adapted that way.
When in Narutothe Nine Tailed Fox's name is revealed as Kurama and the Four Tailed Gorilla's was revealed as Son Goku, some were up in arms, claiming Kishimoto ripped off of YuYu Hakusho and Dragon Ball Z, despite the name of the former existing in Japanese folklore in a relevant tale, that of Tengu Kurama, while the latter is also a reference to Journey to the West, a very pivotal book in Chinese Literature.
When Digimon came out, it was blasted as a Pokemon ripoff because of its use of 'evolution' and monster collecting. Whether or not that Digimon did rip off Pokemon is debatable but it is also moot considering the Megami Tensei series already did the concept of monster collecting (with the use of computers even) long before either did.
Quite a few Western Naruto fans who see a kanchou being performed*
surprising someone by shoving one's index fingers into their butt crack
will assume it's a reference to the "Sennen Goroshi" scene. Kanchouing is actually a popular real-life prank in Japan, the local equivalent to the American wedgie.