Values Dissonance plays a huge part in this category, as Japan has a much higher threshold than Western cultures on what's not acceptable for kids. Any shonen or shojoanime and manga falls under this category, especially some Magical Girl anime. This tends to happen because of the Japanese language's lack of true swearing, which results in some of the harsher words or interjections being translated into English as profanities; therefore, it's not uncommon to find an anime series that routinely uses the equivalent of "damn" or "shit" and was intended for children. If not that, it's usually due to violent content; graphic violence doesn't have anywhere near the social stigma in Japan as it does in the rest of the world (where it has been theorized that blood and gore in the media may desensitize children to committing violent acts in real life).
Similarly, shonen deconstructions, those with Darker and Edgier contents, and those with a pessimistic viewpoint often falls into this trope and is often mistaken as seinen.
Arashi No Yoru Ni is a movie about the arguablyhomosexual romance between two very close male friends of wildlydifferentspecies, and the opening scene features a wolf violently and bloodily getting his ear torn off by a Mama Goat. To say nothing of prey-friend Mei begging carnivore-friend Gabu to eat him near the end. However, it's still a kid's movie — and a fairly gentle-hearted one at that.
Attack on Titan may be one of the most relentlessly serious, dark, violent, and horrifying manga in recent memory, easily on par with some of the grimmer Seinen out there, and yet it runs on Kodansha's Bessatsu Shōnen Magazine.
Azumanga Daioh's sizable Periphery Demographic leads some people to believe it's a seinen series — being aimed at teenage boys, it, along with Dengeki Daioh (where the manga appeared), are actually shonen. And, in America, it sometimes gets lumped in the 18+ section because of the perverted teacher, but, then again, perverted teachers hitting on eleven years old in Japanese Children's shows is just of of those things anime fans have just come to accept.
The anime received an MA◊ rating in Australia purely because of the aforementioned teachers (according to Madman).
Barefoot Gen, a semi-autobiographical manga series best known for its graphic depiction of the nuclear attack on Hiroshima, was originally published in Shonen Jump and aimed at kids, complete with intermittent history lessons throughout.
Black Butler. It has Seinen written all over it, but is published in a Shōnen magazine. It contains violence and gore, murder, child abuse, sex (and in the anime, rape), pedophilia, homosexuality, and questionable dialogue. To top it all off, its protagonists are Villain Protagonists who kill anyone who gets in the way of their goals. On the other hand, cooking competitions, dance lessons, boys put in fluffy dresses, lots of Ho Yay undertones with a cast full of bishonen, and a huge female fanbase might cause it to be mistaken for Shōjo (Demographic).
The author does mention in Death Note: How to Read, however, that the story would've taken a different path if it ran in a seinen magazine, exploring the morality involved in using a Death Note and how society responds to it rather than putting the cat-and-mouse chase between Light and L at the front.
It can be argued that this trope, combined with Values Dissonance, is why Detective Conan (a.k.a., Case Closed!) failed when it was broadcast on Cartoon Network in the United States: it was too childish for [adult swim], yet too violent for Toonami. The aged animation style was the final nail in the coffin. In fact, this anime was aired during family hours in Japan. Complete with brutal murders; complex plots involving suicide, drugs and business dealings; and of course copious amounts of sexual tension.
Case in point: The first episode had two seperate Digimon killing and eating another one, and to make matters worse, Digimon who die that way are never reborn. Other highlights include some of the most horrifying and brutal battles in the series' history, one of the hero's Digimon being eaten and thus permanently killed (by a former friend, no less), someone trying to commit suicide, at least two Heroic Sacrifices, a good chunk of the Digital World and its residences being deleted, one of the Digimon getting stabbed nearly to death and thrown into a sea of deletion while his/her Tamer/s can do nothing but watch, a heart-breakingly Bittersweet Ending, and naked children withBarbie Doll Anatomy. The absolute worse offender, however, is a sweet little 10-year-old girl getting mind fucked by an Eldritch Abomination for at least a week. If the title didn't have "Digimon" attached to it, you'd think it was intended for high school students, but no, it's aimed at kids around that girl's age.
Also Digimon Savers. It like Tamers, less mindfuck, but less shy about detailed violence or blatant fanservice.
Most western fans of Dragon Ball and Dragon Ball Z still insist that it's meant for adults, even though it is very clearly a kids' series.
Fullmetal Alchemist started off as a light-hearted shonen (ignoring the Elric's backstory) with an Antagonist of the Week popping up often until several episodes in. After Nina's death the dark moments got more closer together and more frequent. By the mid-point it hit full Cerebus Syndrome and by the end it resembled mere of a seinen then a shonen.
The anime anthology Grimms Fairy Tale Classics was clearly aimed towards small children. However, it contains many scenes of intense cruelty (such as a princess being falsely accused of killing her baby in the episode "The Six Swans", and Cinderella's stepmother locking her in the attic while she goes to have Cinderella's magic tree chopped down), as well as semi-frequent use of violence and sinister-looking creatures like goblins. The darkest episode was perhaps "Bluebeard", which features the title character killing his wives and nearly kills the most recent one until her brothers save her. Much of the dark imagery was toned down for the English dub, but the show was still quite dark considering its target audience. The catch? This show was aired on Nick Jr., which is known today for very sugary shows such as Dora the Explorer.
Hunter × Hunter (the 2011 version) airs at 11:00 on a Sunday, a time slot meant for kids. Despite this, it somehow manages to br even more gory and violent than most late night shows, especially the recent Chimera Ant arc. Anyone who's able to read Japanese comments for the episodes will find a lot of comment asking how they managed to get away with all the violence in a Sunday morning timeslot without getting taken off the air. However, it is serialized on Weekly Shōnen Jump.
Itsudatte My Santa's original printing was recalled for having the wrong rating on the package, TV-PG instead of TV-MA. Despite that, nobody complained even before its recall, plus that it was serialized on a Shonen magazine.
The American publication of Kare First Love (via VIZ) is rated T (13+) despite Aoi's consistent pressure on his girlfriend to have sex with him. Curiously, the series' rating never increases, even after it displays a rather explicit sex scene and a pregnancy scare resulting from an affair between an adult man and high school teen. Other manga published by VIZ has been marketed to older teens/adults simply for containing frontal nudity — such as Ranma ½.
American parents would be, to say the least, not pleased if they ever saw Keroro Gunsou presented as a kids program, although it's hugely successful amongst kids in Japan. It's even had its own Happy Meal toys.
The anime is a lot more kid-friendly than the manga, though both have the same target demo (probably 4th to 8th graders). Of course, very few kids are going to get the ubiquitous Mobile Suit Gundam references, especially not things like Keroro dressing up like Char Aznable's voice-actor.
Kinnikuman originally started out as a parody of Ultraman complete with action and comedy being somewhat expected of a Shonen manga. Then, the series shifted from being just a parody of superheroes to a series about Professional Wrestling. Despite the comedy remaining in the series, there was notably a lot more violence in the show. For one, the wrestling matches couldn't really be called matches anymore, as whenever someone won, it usually meant that they won via killing the other person, complete with Family-Unfriendly Death for the defeated (One person actually got eaten alive by their opponent). Keep in mind that it was still considered a Shonen manga after the shift. The anime tried to tone down the violence, but a lot of it was still kept in.
The same can be said for the anime of its sequel Kinnikuman Nisei (known in the US as Ultimate Muscle). Despite being aimed at kids, it still follows in the footsteps of its predecessor with violence and loads of innuendo in both the original and the dub. The manga averts this trope, as it's marketed for adults.
Life may be a Shoujo manga however it's considerably more mature in nature than most shoujo. From the graphic nudity, sexual scenes, and gore.. It's no wonder Tokyo Pop changed it from being for "Older Teens" to "Mature". One scene in volume 6 involved showing two characters somewhat graphically having sex. It can easily be mistaken for a Josei manga.
Maicchingu Machiko Sensei is an anime about a quirky teacher who helps her students through everyday problems such as bullies and school work. Sometimes, she takes them on wacky adventures as well. It was a show aimed at middle school children. Oh, and every episode featured the main character naked due to her students' pranks. And yes, Barbie Doll Anatomy was completely averted.
Eiichiro Oda repeatedly claims that his series One Piece is aimed at children. It is read by people of all ages in Japan and mostly young adults in the west. Oda does a pretty good job at keeping it family-friendly, however, counting aside the violence and Fanservice that's become standard in anime.
Pandora Hearts is a shonen manga series similar to Black Butler. Like Black Butler and Death Note it probably would be more suited for a Seinen magazine, due to its violence, characters with violent and complex pasts (quite a few involving Eye Scream)and generally being quite a mature manga series.
Pokémon Special is in fact classified as a kodomonote children's manga. However, its strong sense of continuity and characterization, as well as frequent use of violence and even occasional deaths, has people convinced it's meant for an older crowd.
Of course, the main reason WHY it's classified as kodomo is because the mangas are based off of a Merchandise-Driven children's video game.
Toei held two all night Pretty Cure events at a movie theater for grown ups only due to a Japanese law that doesn't allow children to see movies after 8:00PM. They showed some Pretty Cure movies during this marathon.
The show itself is primarily known for its abundance of martial arts fighting, not normally seen in magical girl anime aimed at a young audience.
Read or Die Rehabilitation is more risque than its Seinen counterparts Read or Die and Read or Dream, yet is serialized on a Shōnen magazine.
Sailor Moon. No, it was not only for teens in Japan; watch the commercials. The first season finale deserves special mention where four of the Sailors are brutally killed (they get better). Reportedly so many children were upset about the deaths that they made themselves sick. For the US dub, the deaths were censored by implying that they were merely being held prisoner and the two part finale was edited into one episode.
Saint Seiya was aimed to children in Japan, as well in Europe and South America, having a lot of controversy due the huge amounts of violence, frightening scenes and religious/mythological references.
Science Ninja Team Gatchaman has a gender-bending villain, plenty of violence and death, and (in the first series) TWO attempts to destroy the planet. Its target audience was 6-year-olds.
The majority of anime produced in Japan, is meant for people aged 6-17 (Animation Age Ghetto is just as strong there as in America, so you'd hide your anime posters just as well). The reason many Americans believe the medium is primarily targeted to adults is because cartoons in their country often have little violence, blood, sex, or swears. In Japan, the censors and ratings are arranged a tad bit differently then in most western countries.
In general, the term anime means nothing more then "cartoon" or "animation" in Japan.
Blockbuster once had a point where they rated every anime "Youth Restricted Viewing", even relatively kid-friendly titles like Tenchi Universe, Project A-Ko, K.O. Beast, heck, even Grave of the Fireflies was getting slapped with this rating; basically, every anime that wasn't Pokemon; however, Blockbuster's system for the "Youth Restricted Viewing" rating was all messed up; on your account, you either had to allow it all or none of it. So, you either can have none of the anime section, or you can have all of it, including the hentai, with the same rating, showing no difference between the two; you could end up with a "Youth Restricted Viewing" title that has a "damn" or two and maybe a drop of blood, or you could end up with an also "Youth Restricted Viewing" hentai that has everything but the kitchen sink in it, with no warning. Rather difficult system if you've got a young kid into anime, so it's no wonder they went out of business.
Blockbuster had this problem in general. It was especially galling that drama and horror were filled with soft-core porn that a child could rent with no restrictions, but project A-Ko was restricted. Complaints fell on deaf ears and employees could do nothing because corporate was insane.
Yoshiyuki Tomino's early career is littered with examples of shows that were marketed to kids but were not so kid-friendly in their content. His Zambot 3, which earned him the nickname "Kill 'em All Tomino", looks like a typical Saturday Morning cartoon, with a bunch a school age kids saving the day with a colourful Combining Mecha, but quickly turns into something much darker; the property damage and civilian casualties are depicted much more realistically than in the typical Super Robot show, the public turns against the kids and tries to murder them several times when it becomes apparent that they're the whole reason Earth is getting attacked in the first place, and most of the main characters die a bloody death in the finale. The original Mobile Suit Gundam had a similar effect at the time it first appeared, as the term Real Robot hadn't been invented yet and Humongous Mecha were still widely considered the stuff of children's programming, Mr. Tomino's earlier work not withstanding. And let's not even mention Triton Of The Sea or Space Runaway Ideon...