Inverted with Dr. Seuss books. For instance, in Hop on Pop, Dr. Seuss included a line "When I read I am smart / I always cut whole words apart. / Con Stan Tin O Ple, Tim Buk Too / Con Tra Cep Tive, Kan Ga Roo." to make sure his publisher was actually reading the manuscript. (He was, and the line was changed, as Seuss planned)
In Dork In Disguise by Gordon Korman, Jerry says it would be pleasant bumping into his crush, Cinnamon, in the hallways.
Phenomena started off branded as from ages 10-12. After the 1st book it got constantly darker, something the publisher failed to notice and even put it on their 9+ list. The most noteworthy is that in the 1st book is there a really cheerful girl who one comes to like a lot. When one meets her again in the 6th book, things have gone badly for her and her people. Given her personality, one would think she would be strong and support the children. Instead, she's awfully silent, and hates being touched, implying that something awful happened. To make matters worse, she was underrage at the time, to drive the implication home is does the abuser say while acting all creepy around her:
I almost fell in love with her myself.
The most noted example might be Animal Farm. Designed to be a criticism of Stalinism, it had great difficulty finding a publisher, largely because of fears it would undermine the World War II alliance between the US, UK, and Soviet Union. If it had been written "straight", it might not have been published; as an allegory about farm animals, it could slide by.
Where's Waldo?: Cartoonist Martin Handford hid his titular hero amongst massive (sometimes absurdly so) crowd scenes in which so many unusual events were taking place that you had to look carefully to make out the guy in the striped shirt and ski cap (which was the point of the book). Many of the events depicted were ridiculous or bizarre, and several of them were controversial inclusions for a book aimed at kids ages 6 to 14: a vacuum cleaner sucking a woman's dress off of her body,a man graphically vomiting, and another guy getting accidentally hit in the nuts. One of these sneaky scenes was so subtle that you might not even notice it: a boy at the beach teases a bikini-clad beauty by placing the cold end of his ice cream cone on her back, causing her to bolt up from a prone to a semi-prone position; unless you're looking closely, you might not notice that the girl has loosened her bikini top so as not to get tan lines while sunbathing, and she's about to expose her bare breasts to the world.
Either Christopher Paolini is unaware of the meaning of such things as comparing the size of, ahem, "bruises" while your pants are down, noticing an elvish groin is hairless, and the like, or he's a master of Ho Yay.
In the second book of the series, the word "slattern" is used, and another time a character is talking about relationships between men and women and says a man's "loins" can make even the most sensitive man a dribbling fool or sly fox. The word "Loins" means genitals. And don't forget the huge amount of alcohol use and references in the series, and there's even a scene with smoking of some kind of weed.
Not to mention Eragon and Arya getting drunk/high on spiked alcohol to unwind after a particularly stressful battle in book 4.
Brisingr, the third book of the Inheritance Cycle, has a moment during one of the chapters from Saphira's POV. She tries to talk to Roran telepathically...
Saphira? he asked.
Do you know another such as me?
Of course not. You just surprised me. I am...ah, somewhat occupied at the moment.
She studied the color of his emotions, as well as those of Katrina, and was amused by her findings.
The fact that a character states his parents died of "the pox". Yes, it can be a generic term for a disease, but it nearly always was actually used to mean syphilis. This is another one of those where you're not sure whether Paolini misused the word or was deliberately Getting Crap Past the Radar.
Gulliver's Travels by Jonathan Swift was written mainly with the goal of making up outrageous fantasy countries to satirize existing ones without getting reprimanded by the censors, the way he would if he criticized them directly.
He tested exactly how much crap could get past by naming a country "Laputa". Basically, Spanish for "The Whore". Hayao Miyazaki once noted that if he'd known what it meant, he wouldn't have named one of his films after it.
The first chapter has Gulliver talking about his old apprenticeship days under Dr. Bates. He often mentions [his] "good MasterBates".
Mystery author Dashiell Hammett used to enjoy putting things in his books that sounded like they might be dirty, just to annoy his editor who would cut them out before the books were published. But his editor did not notice the word "gunsel" in The Maltese Falcon, so it was included in the published version. The editor thought the word meant a "gunman", and many American writers imitating Hammett use the word "gunsel" to mean "gunman". In fact, the word meant "homosexual lover (particularly one who is the passive partner to a prisoner)". "Gunsel" managed to survive in the 1941 film as well. (Although Peter Lorre's character makes up for it with the perfumed white gloves.)
There is also a strong implication in both novel and 1941 film that Peter Lorre's character Joel Cairo and the "gunsel," Wilmer, are having a homosexual affair.
This line from the novel, about the gunsel, is clearly attempting to slide something past the censors: "The boy spoke two words, the first a short guttural verb, the second 'you.'"
One of the characters in Stanley G. Weinbaum's 1934 classic A Martian Odyssey is named "Putz".
During The '40s, writers for science fiction magazine Astounding made a game of getting dirty references past bluenose assistant editor Kay Tarrant. George O. Smith succeeded with a reference to a tomcat as "the original ball-bearing mousetrap".
Robert A. Heinlein's The Star Beast, written for what would now be called the young adult market, stars John Thomas Stuart XI, latest in a series of custodians of the titular alien pet. In the end, it is revealed that the pet is a) female, b) royalty, and c) considers the Stuart line to be her pets. Heinlein managed to get away with writing of Lummox's "hobby of raising John Thomases".
James Branch Cabell's Jurgen, a comedy of justice details a medieval character's multiple marriages, affairs, and a full-on Crowley "Gnostic Mass," complete with ritualized deflowering and child (no longer-) virgins. In 1919, and went on to defeat a number of public indecency lawsuits, proving that Cabell, like his Jurgen, was indeed a "monstrous clever fellow."
Damon Knight's short story "Cabin Boy" has the titular character "circumnavigating the skipper", referencing the bawdy shanty "The Good Ship Venus". At least one collection included the text of the shanty, assuming readers may not have heard it.
A Series of Unfortunate Events: You have to be really sharp to catch this one, but in Book the Fifth, the three siblings are forced to work as a secretary (Sunny) and study an impossibly vast amount of material for a test (Violet and Klaus). At one point while Sunny is frustrated with her work (making staples by hand), she says "Merd" as an exclamation of frustration. It's awfully close to "merde", which is French for excrement.
In book #13, Count Olaf quotes "Man hands on misery to man/it deepens like a coastal shelf/get out as early as you can/and don't have any kids yourself," from Philip Larkin's "This Be the Verse." The opening line of the full poem is "They fuck you up, your mum and dad." What was the target age group for these books again?
Also, the fact that in the last one, the coconut cordial is made by fermentation, has a 'strange, strong' taste to it, and contains "an opiate". Snicket was OK mentioning alcohol before - Olaf had a ton at the carnival - but here the children are drinking it! Even Sunny Baudelaire!
Back in Star Wars (Marvel 1977), Leia tells Dani (a Zeltron and therefore Luke fangirl) something along the lines of "I didn't know what you do was illegal in that many systems." Yes, she basically calls Dani a whore.
Also from Marvel, when they visit Zeltros, Luke has a legion of fangirls, and in one scene a fanboy.
The X-Wing Series. Stackpole became known as the "sex writer" for a long time. He hints quietly compared to more recent works, but brothels get mentioned, it's obvious that Erisi Dlarit wants to have sex with Corran, and there's a point where it's implied that people think three characters are a threesome.
Teneniel Djo declares Luke her slave in The Courtship of Princess Leia.
The title Emperor's Hand led to many a masturbation joke among fans, though it probably wasn't Zahn's intention.
The Black Fleet Crisis. Not only does Before the Storm have naked Luke, but he has a one-night stand at the end of Shield of Lies, which lampshades Luke's role as Chick Magnet.
New Jedi Order: Anakin poses as Mara's sex slave for a mission, and she notices a bulge in his pants, right below his belt. Mara concludes that it's a Stokhli stick. For those not familiar with the Expanded Universe, a Stokhli stick is a stick-shaped weapon that shoots goop all over its target. Worse yet, while this started as a joke, there actually are Anakin/Mara fanfics, including ones where Luke approves of their relationship.
"I hope he doesn't call you Master the way I call you Master." -Mara Jade Skywalker
Jacen on the cover of Destiny's Way.
Jacen naked and being tortured. Fetish or nightmarish, depending on your point of view.
Ben getting molested by Tahiri.
One line in that chapter where Ben gets molested is commonly thought to be silly, the "big, strong Jedi Knights" line. It can be interpreted that it is something much, much squickier than a throwaway compliment: Tahiri would seem to be complementing Ben's...penile size.
Troy Denning has hinted to Three-Way Sex between Jaina, Zekk, and Jag at least twice.
This series has a lot of tentacle usage (ostensibly Combat Tentacles). Ben's love interest, the 16 year-old Vestara, gets heaved up in the air and repeatedly impaled by them. Later, there's an extended scene where tentacles immobilize her, and blind her, while she argues with Ben. The kicker, has to be what happens in Vortex , with a feminine Eldritch Abomination, who has tentacles instead of fingers. She grapples and suffocates Luke for a while. Then, another man suckles from her other set of tentacles like a hungry puppy... All but the first was written by Denning.
Speaking of Vestara, Luke tells Ben to be prepared for a betrayal when dealing with her, and to not do anything that Luke wouldn't do. Vestara is a Sith. Mara Jade was a Sith when Luke met her.
Allison has Luke talking about how well-muscled Ben is.
Christie Golden gives us this exchange:
Ben: "...I think I am in dire need of a sanisteam."
The Revenge of the Sith novelization by Matthew Stover is full of them. The part where Obi-Wan wakes up hanging on Anakin's shoulder? Yeah, that more than suggests Ho Yay between them. And Artoo suggesting where Anakin could look for a datajack couldn't have been any more made of this trope.
Doctor Who and the Dinosaur Invasion, Malcom Hulke's novelisation of his television story "Invasion of the Dinosaurs", fairly obviously made the Anti-Villain Henchman to that story's main Anti-Villain into a gay man, which the original serial had (for fairly obvious reasons) not even implied. Pretty daring for a mid-1970s book intended for children. This reinforced an Aesop present in the original story but emphasized in the novelisation that the villains consisted of emotional cripples who wanted to Ret-Gone away the present world because they just didn't fit in with the current one. (Again, gay people found it harder to gain acceptance in those days.)
In Coraline, the Other Mother touches the heroine quite often. And in a way that the girl reallyhates. And the phrases "You know I love you..." and "Come out when you learned to be a loving daughter." Add in the premise - a girl neglected by parents finds some other people who give her much more "attention" in a way described above and has a hard time getting away. This all sounds more like child abuse than a typical fantasy story. So we get a children's book that's a thinly-veiled story about child abuse.
In Ender's Game, the insectoid alien race is called the buggers. Accordingly, anyone who opposes the war (or is just acting like a jerk) will often get called a "bugger lover." Just in case this was too subtle, the sequels introduce a more "formal" term for the aliens — formics — and make sure to note that it has become the more accepted term in most places because "bugger" is considered a naughty word by the British.
The How to Train Your Dragon series gets quite few things past the radar with Dragonese, which includes words like 'piss', a variation of 'crap', and 'cack'. In a series meant for children.
In the first book there is a picture of Gobber with a tattoo of a mermaid on his butt that's topless, complete with "details".
In Martin the Warrior, when Martin is staying at Noonvale, Rose offers to show him his room, leading her mother to say: "No, I'll do it, you'll have him up all night talking." Talking? Really? (Well ...)
In Viking It and Liking It, there's a bard named Bullshik.
There's a lot of this in Colin Thompson's "The Floods" books. A perfect example can be found in Prime Suspect, when Mordonna tells her husband that she could turn him into a girl "with a couple of spells and a pair of scissors." Ouch.
Robin Jarvis' The Dark Portal (the first Deptford Mice book): Morgan makes a snarky comment to the old fortune-teller Madame Akkikuyu along the lines of "I knows wot you were afore you got too old'n'ugly."
Jarvis got quite a bit of crap past the radar in A Warlock in Whitby, the second book in his Whitby Witches trilogy. In addition to Jennet's rather questionable crush on Nathaniel Crozier, there's also the matter of Nelda (at seventy years old, a young adolescent by aufwader standards) being made to marry tribal elder Esau. Not only that, Esau forces Nelda to consummate their marriage in exchange for information which could save their tribe, leaving her pregnant. Because the curse on their race means pregnancy is effectively a death sentence for female aufwaders, Nelda appears to be doomed.
In the book Fudge-A-Mania Fudge mentions that he sleeps with his mother when he's afraid at night. Sounds innocent enough right? Well, he tells two elderly characters (who get married in the end) that they should sleep with each other followed by a "Fudge!" Late to the Punchline indeed!
Later, when they announce their engagement, Fudge asks a lot of questions.
Fudge: All I know is that you get to sleep in the same bed.
Buzzy Sr.: That's the best part.
Fudge's Mother: Oh, Buzzy...
In Marco and the Tiger, a book for young adult readers by John Foster, published in 1966: In the second to last chapter of the story (at which point many bored readers might not have been paying attention), a zookeeper at the Audubon Park Zoo in New Orleans tells the titular hero that he can't accept his pet tiger because he's not allowed to take in animals that haven't had background checks ever since a rogue gorilla "[busted] the Mayor....on the nozzle!" Given the angry way the zookeeper says the line, I'm pretty sure that "nozzle" there doesn't mean "nose."
In one The Berenstain Bears book, where the boy gets lost at the mall, he gets to look in the lost and found. One of the items looks suspiciously like a condom.
In the children's book Mungo and the Picture Book Pirates, at one point Mungo is taken by surprise and exclaims "Oh, Christopher Columbus!"
Richard Adams' Watership Down introduces words in the rabbit language, sometimes to express concepts that are only meaningful in their world (like tharn, a rabbit frozen in fear,) sometimes just to give an alien atmosphere. But it also allows Bigwig to more-or-less tell his enemy to "Eat shit!" in what was originally marketed as a children's book. It's subtle, because even this has a shade of lapine meaning. Rabbits normally do eat shit in the burrow, to recycle undigested cellulose: but what Bigwig says is "silflay hraka," literally "eat shit outdoors," which makes it insulting.
The success of this probably inspired Adams to use the same trick in his very adult fantasy novel Maia. In some amazingly pornographic scenes for a mass market paperback in the 1980s, he got away with explicit descriptions just by putting the sexual nouns and verbs in the Becklan language. He may have been inspired by the 1970s translations of The Kama Sutra and The Perfumed Garden, which left the terms "yoni" and "lingam" untranslated so that the publishers could pretend there was some doubt about what was being described.
In one of the comic sections of Malice, the artist snuck in pictures of the body of a naked woman on top of the mort-beast, bare breasts with nipples and all. One wonders how nobody seems to have noticed this little fact, since the book is labeled and sold as a book for the 9-12 age range.
In The Message, Marco mentions having "weird dreams about that woman from Baywatch." Now what kind of dreams might those be...?
In the Star Trek: Deep Space Nine Relaunch, Nog refers to another character as a "cold-hearted Moogi-jokk". Seeing as we know that "Moogi" means "mother", we can work out what he's saying.
In the children's book The Anti-Peggy Plot, where three kids attempt to sabotage the efforts of their soon-to-be Wicked Stepmother, they sneak into what they think is her apartment. It turns out that it's the wrong one, and they learn this when the couple who lives there comes in, the man saying "Damn, am I in the mood." It was a few years before this troper realized exactly what the man was in the mood for.
In The Magician's Nephew, when Uncle Andrew calls Jadis a "dem fine woman", the author is using a Funetik Aksent to disguise the fact that he is calling her a damn fine woman.
In The Silver Chair, the author once again covers up a curse, this time with a curious contraction, in which Jill says something is "Dam' good of [Eustace]".
In The Amber Spyglass (controversial for its atheistic theme, but marketed as a children's book), when Mary is telling Lyra and Will about love, Lyra feels an interesting sensation...
"As Mary said that, Lyra felt something strange happen to her body. She found a stirring at the roots of her hair: she found herself breathing faster. She had never been on a roller-coaster, or anything like one, but if she had, she would have recognized the sensations in her breast: they were exciting and frightening at the same time, and she had not the slightest idea why. The sensation continued, and deepened, and changed, as more parts of her body found themselves affected too. She felt as if she had been handed the key to a great house she hadn't known was there, a house that was somehow inside her, and as she turned the key, deep in the darkness of the building she felt other doors opening too, and lights coming on. She sat trembling, hugging her knees, hardly daring to breathe, as Mary went on."
The pivotal moment when Will and Lyra finally realize that they love each other, immediately breaking into a storm of ridiculously passionate kissing. Though not actually stated, it leaves the reader wondering if they had sex. They wouldn't know what they were doing, obviously, but the whole scene is basically written to show that they have begun the transition to adulthood, and sex is a staple literary symbol for the end of one's childhood.
The whole concept of touching one's dæmon. It's normally regarded as an instinctive taboo, but in The Amber Spyglass Will and Lyra, sometime after realizing their love for each other, accidentally touch each other's dæmons. To their surprise it brings a sort of sensual pleasure inside of them. In The Golden Compass, however, when one of the Bolvangar employees grabs Pantalaimon, she feels profoundly violated. This could mean that the act of touching another person's dæmon is sexual in nature, and that when lovers do it, it's pleasurable but when a stranger does it, it's as bad as rape. It could easily be surmised that Will and Lyra had sex (again) immediately after doing this, because the scene cuts at that moment.
And then there's that scene in The Subtle Knife when Ruta Skadi tells the group about finding Lord Asriel in his bedchamber during her exploration of his fortress. She then skips to her conversation with him afterward, but the narration states that all the other witches knew what she and Lord Asriel did but Will and Lyra knew nothing of it.
Australian poet Gwen Harwood, whose work was published in the magazine Bulletin, wrote two poems. Reading down the first letter of each line you got the message 'So long Bulletin. Fuck all editors.' Somehow no one at the paper noticed this and the issue had to be recalled.
In The Vampire Files, set in 1930s Chicago, an in-universe example would be the art of Evan Robley. From nearly any angle and distance, his paintings appear as colorful, abstract works respectable enough for the sensibilities of any censor of that era; when viewed from exactly the right spot, however, they're revealed to be detailed self-portraits of Evan's favorite body part.
An in-universe example in Erica Jong's How to Save Your Own Life (not for kids!). Swear words, obscene words, body parts/functions etc. aren't allowed on vanity license plates. Isadora has a license plate that says "Quim," which is an (obscene, comparable to "cunt") old English term for vagina.
In The Good, the Bad and the Mediochre, Mediochre Q Seth gets indignant when one of the Mooks surrounding him at one point refers to Charlotte as his "girlfriend". He replies that they've "only known each other for a few minutes" and adds that he's "not talking Biblically. Or like The Crucible". Both The Crucible and the King James Bible use the word "known" as a euphemism for "had sex with".
The Mark Twain story "The Story of Grandfather's Old Ram" features a narrator who gets so sidetracked he falls asleep before finishing his story. That's fortunate, because the story appears to be about how his grandfather was given the gift of unexpected sex when he bent down to pick up a coin.
Presumably the only reason the childrens' picture book It's a Book was able to get away with using the phrase "It's a book, jackass!" is because the character being referred to as such is a donkey.
Circle of Magic mentors Lark and Rosethorn have a very close friendship throughout the series, referring to each other by nicknames and hugging a lot. Sequel Will of the Empress has the protagonists maturing and is for an older audience as well... and Briar refers to Lark and Rosethorn as lovers. At which point the reader goes back to the first four books and wonders how they missed it.
"Been spending a lot of time keeping each other warm?"
In the second book, Dr. Bergstrom lets David use his good luck talisman for, well, good luck. When he discusses the matter with Zanna later, he asks her, "Did you shake his totem?" This elicits a shocked "Pardon?!" before David explains himself.
Humpty Dumpty: Seven years and six months! An uncomfortable sort of age. Now if you'd asked my advice, I'd have said "Leave off at seven" - but it's too late now.
Alice: I never ask advice about growing—
Dumpty: Too proud?
Alice: I mean that one can't help growing older.
Dumpty:One can't, perhaps, but two can. With proper assistance you might have left off at seven.
Yes, let's get into discussing murder and suicide with a seven-year-old girl.
Life With Father (1935): The very repectable stockbroker Clarence Day Sr. had no qualms about using "damn", "damned" and "damnation" in front of his children. In the 1890s. It got past the radar (children were not discouraged from reading it) because the book is autobiographical (written by his son).
In Moon Rising Winter threatens to freeze Moon's body parts and snap them off, one at a time, if she doesn't do what he says. In a book for eight-year-olds.
In The Lost Heir, Starflight is oddly eager to have Sunny, his crush, ride on his back.
The Hidden Kingdom when the Dragonets visit the Ice kingdom, Clay jokes about being the one to sleep next to Sunny, since she, as a SandWing, exudes heat. Starflight who's in love with her looks oddly crestfallen afterwards.
A major plot point in Escaping Peril is that Arctic was brainwashed into committing suicide by, explicitly, disembowelling himself. (That method's unnecessary sadism is the main reason why the protagonists disagree about whether or not the perpetrator should be freed.)
Legends: Darkstalker makes it even worse, revealing that Arctic not only did it solely with his own talons, but he also tore his tongue right out of his mouth in the process.
In the novelization of BIONICLE: Web of Shadows, in a scene cut from the animated feature, Nokama and Matau discuss having bestial urges, like building nests. Matau, who was previously shown to have a great affection towards Nokama, inquires whether she has any "urges" that involve him, before the scene ends. In a later story from another saga, Kalmah apparently finds the properties of organic squid, such as their method of breeding, fascinating.
At one point in The Princess Diaries, the teenage protagonist talks about how she wouldn't mind it if her boyfriend were more possessive of her. This sounds innocent in theory, but the way it's written comes off like a metaphor for kinky sex.
In Amber Brown Wants Extra Credit, the title character comments "Come on, everyone, let's turn on the oven and do some pre-heating." This causes Amber's mother and her boyfriend Max to laugh, but when Amber asks what for, she's told that it's a private joke. This causes her to rage to herself about this sort of adult hypocrisy, as whenever kids tell a joke that they want to keep private, they're made to tell anyway, asked "... would you like to share that with the rest of the class?" and if they don't, they get detention.
In Robert McCloskey's Centerburg Tales the illustrations accompanying one of Grampa Hercules' tall tales include two depictions of a naked Grampa Hercules in a creek (with his groin area hidden by the water) and a rear-view depiction of his equally-naked prospecting partner Hopper McThud.
Jinx High has an in-universe example. The novel is set in the Bible Belt, and the high school prom opens with a Maypole dance. Diana and a minor character who was a hippie in the 60s both recognize the signs of an accurate fertility-ritual Maypole and have to hide laughter. Until Diana realizes it's the full Sex Magick version instead and finds herself running mystic damage control.
Lottie and Lisa being a children's book written in the 1940s, it's very subtle, but there are some instances of adult themes being referenced:
The painter Gabele throws a cloth over the painting he is working on when Lotte-as-Luise comes to visit him because scenes from classical antiquity are not always "appropriate for children". Made hilarious in the 1950 movie as he does hide the painting he is currently working on... only to have several other paintings of naked women around the flat.
It's mentioned that when the parents were still married, the father often left the mother alone with the twins to compose... and "practise singing" with pretty opera singers. She apparently didn't think much of it and asked for a divorce.
Gangsta Granny has a joke about the mother thinking the son is hiding a "naughty magazine" under his bed.
Topsy And Tim: In one of the books, the titular kids and some friends are taken to the circus by their Dad. After the show, they visit one of the clowns in his caravan. He is very welcoming, and gives all of the kids a big glass of lemonade, but "Dad only got a little drink". In other words, as any real-life Dad, reading it to his own kids, would realise, Dad got a whisky.