Arthur C. Clarke's 2001: A Space Odyssey directs the fateful mission to Saturn, but The Film of the Book goes to Jupiter (according to The Other Wiki, because the special effects team couldn't make a satisfactory ring effect for Saturn). When Clarke wrote 2010, he decided that Jupiter worked better for the new plot and went with it.
3001: The Final Odyssey retconned the events of the first 3 books to occur at a later date. 2001 was changed to 2015, with 2010 and 2061 getting a respective forward push, as well. (It was written in 1996, by which time it was pretty clear that Clarke's 2001 would not happen.)
Word of God — erm, I mean word of Deus — is that each book takes place in a slightly alternate continuity from the others. Which explains Dave/Star Child's Badass Decay between 2010 and 2061, and Floyd not appearing as part of the Dave/HAL/Monolith entity at all in 3001. Some people still find this incredibly lame.
In the third Alex Rider book, Skeleton Key, there is a villain called Conrad who is stated to have been blown up by a bomb whilst they were carrying it, and surgically put back together again. Author Anthony Horowitz appears to have later decided this was a little extreme, as a later book mentions Conrad but changes it to a bomb in the boot of his car detonating whilst he was driving it to an army base he was planning to plant it in.
In Animorphs, Ax is at the beginning unable to communicate in thought-speak while in human form, explained by the fact that humans cannot use thought-speak. Later in the series, Ax repeatedly uses thought-speak while in human morph, because it is, after all, a morph.
In the first book, Jake can communicate in thought-speech with Tobias when he's in a morph and Jake isn't, which would have come in really handy for the characters later. K.A. Applegate intended to take that scene out, but forgot.
An in-universe explanation popular in some fanon circles is that humans possess a vestigial capacity for thought speak similar in function to the Andalite sense of taste. Neither Human nor Andalite scientists ever noticed it and the Yeerks weren't interested, but it helps explain why humans innovate so quickly. Under this explanation, the above example worked the way it did for three reasons: Jake has a strong personality and is unusually able to 'project' his thoughts, Tobias has a sensitive personality and is unusually able to 'hear' thoughts, and both of them expected it to work - but since it didn't work for anyone else, they both later thought they were imagining it.
In Arrivals from the Dark books, there's plenty of discontinuity between the primary series and the Trevelyan's Mission spin-off series that takes place half a millennium later. There's also some discontinuity between the books of the primary series, such as the existence of annihilator-armed ships in the HaptorSpace Navy (a plot point in book #3). Later on, though, in a book that takes place after a human-Haptor war, it's stated that Haptors have never actually developed Anti Matter technology to arm their ships with annihilators, which is the main reason why humanity beat them so soundly. There are other details, such as the claim that kinetic and missile weapons have been phased out on Earth after the development of Deflector Shields, which make any such weapons useless (indeed, dangerous to the shooters, as they can reflect the shots back at them). Then, later on, it's revealed that, yes, missiles are indeed still in use aboard ships.
Early on, Jill, a member of the We ♥ Kids Club, is established as serious and thoughtful; at one point, Dawn describes her as being like Mary Anne. In the first California Diaries book she is portrayed as very childish, which contributes to Dawn, Maggie and Sunny drifting away from her.
The Brewer children's mother and stepfather are named as Sheila and Kendall in an early book, later retconned to Lisa and Seth when they feature more prominently in later titles.
Similarly, Mary Anne's late mother was named Abigail in the fourth book, but later books identify her as Alma. This is also fixed in reprints.
There was a short spinoff series where each of the girls writes an autobiography. They must have been written by different writers, because Kristy, Mary Anne, and Claudia have conflicting memories of their elementary school years (when they all knew each other).
In the early books, Dawn is a semi-vegetarian who eats chicken and fish; she avoids red meat because she thinks it's unhealthy and doesn't like the taste. She specifically says in one book that her vegetarianism DOESN'T have anything to do with feeling sorry for cows, and in fact she doesn't even like animals all that much. This is somewhat hard to reconcile with the radical environmentalist she is in the later books.
The prequels to The Belgariad and Mallorean books do this to several different plot points. The most egregious example is that taking into account Silk's cameo towards the end of Belgarath the Sorcerer, when he and Garion encounter Asharak in Pawn of Prophecy, Silk should have recognised Asharak on sight, should have known that "Asharak" was one of Chamdar's aliases, and should hence have realised that the fact that Asharak was poking around was highly significant and should be reported to Belgarath immediately. It's handwaved with the implication that Asharak was tampering with his mind, but that still doesn't explain why Belgarath didn't react to the name.
In BIONICLE Chronicles #1: Tale of the Toa, the Toa defeat their shadow clones by taking on each others' duplicates and using their own individual tactics. The 2005 Encyclopedia later explained that the Toa actually defeated their own doubles by absorbing them and accepting that darkness is a part of their being. Other parts of the novel were later reinterpreted as well, in part to make up for the absence of the climactic showdown with the Big Bad — in the book, the shadow doubles are the final foes, not merely the lead-up to the Makuta.
An in-universe example: in The Chronicles of Narnia: Prince Caspian, the "history" under Miraz's rule is described as "more dull than the driest history book you have ever read, and less true than the most exciting adventure story."
Since the Cthulhu Mythos has had so many authors over so many years, lots of stuff gets retconned back and forth. Particularly notable is August Derleth's oft-reviled attempt to impose Elemental RockPaperScissors and a good/evil dichotomy on Cosmic Horror and fans revising the wings of Elder Things and Mi-Go (which were originally said to allow them to fly through space) to solar sails.
Discworld, by virtue of being a Long Runner, has a few discrepancies (including when the books actually take place) between the earlier books (such as Wyrd Sisters) and the later ones. Then Terry Pratchett went and justified it all in Thief of Time by explaining that the History Monks just take any time they need and dump any leftover time they have into the ocean. Then there's also the matter of alternate histories...
Discworld history had actually been shattered by the trapping of Time, creating the discrepancies.
In a bizarrely creepy in-universe example, this is actually how the Auditors of Reality COMMUNICATE. They don't talk; they just subtly change the past so that they HAD talked.
A key example from the Discworld novels is the way the key scene in Mort where Mort and DEATH fight a duel is revisited in Soul Music to incorporate the unseen presence of Susan Sto Helit (stepping back to a past before she was born, in order to discover why DEATH spared her father's life).
The Discworld maps have had to be ret-conned in later editions to accommodate things which were presumably there all along but which had simply not been conceived in those novels, prior to the release of the first city map. This is all oddly recursive. Similarly, the Discworld Map had to be revised to incorporate whole new countries not there in the books at the time, or to reshape older named countries whose backstories changed and evolved.
Don Quixote. Part 2, which was written a decade after the publication of Part 1, is predicated on the idea that Part 1 was published as non-fiction in his world. This gives the author an opportunity to address contradictions in the original work, by having a character who had read it ask Quixote himself for explanations of what really happened.
The actual explanation for this is that there was a unlicensed second part "Second Volume of the Ingenious Gentleman Don Quixote of La Mancha: by the Licenciado (doctorate) Alonso Fernández de Avellaneda, of Tordesillas" written that directly insulted Cervantes. It ended up serving as the impetus for Cervantes' Part 2, and there are many jabs, both direct and indirect, at this spurious sequel, in Cervantes' own sequel.
The Dragonlance War of Souls trilogy retcons the role of Chaos, "Father of the Gods" to "delusional god who thinks he's father of the gods."
Not to mention retconning out the ending of Dragons of Summer Flame. Not only does Tasslehoff not die, but Takhsis steals the entire world, somehow pretending to be Fizban while departing with Raistlin. This couldn't have happened, because Raistlin somehow ends back with the gods, a major plot point in Dragons of a Vanished Moon.
Officially, there's now another Chaos retcon. He's an Eldritch Abomination equal and opposite to the Highgod; while not the ultimate creator, the universe was, in a sense, created from him, and he is as far above the gods as they are above mortals.
Another retcon is the conception of Steel Brightblade, which no character knew about until The Second Generation.
There are a number of inconsistencies between Ender's Game, the sequels, the sidequel, the sidequel's sequels, and the prequels. A large part of it has to do with technology and how it works. For example, in the original novel, the previous Bugger/Formic invasions were fought with "primitive" weapons like missiles, with the MD Device (AKA the Little Doctor) only being developed in anticipation of the Third Invasion. Additionally, a number of technologies are stated to have been reverse-engineered from the Formics, including Artificial Gravity and Faster-Than-Light Travel. While this latter fact is not contradicted by the sequels, the nature of FTL travel changes dramatically over the next several books, from a barely-understood "Park shift" that seems to instantaneously accelerate a ship to relativistic speeds to a drive that takes a long time to achieve the same (that's right, technology seems to get worse over the millennia for no reason other than to provide a reason for an admiral to do something questionable). The prequels (set during First Contact) forget all that and make humans more advanced than stated in the original novel, with Artificial Gravity already being a thing on Earth (in fact, the Formics are stated that they've never had it), and the Formics less advanced (yes, they're advanced enough to curb-stomp the unprepared humanity, but it's implied that they only dominate the Second Invasion mostly through numbers and the fact that humans haven't had time to build enough warships). The prequels retcon the Formic interstellar drive to a Ram Scoop. It was also originally implied that the Formics nuked China during the First Invasion from orbit, when one of their scout ships broke through the blockade. In fact, humanity didn't even have a defense fleet at this point, according to the prequels, and it was Asteroid Miners fighting the ship. The single mothership entered Earth's orbit and send down landers. No nukes were used, since the Formics were more interested in terraforming the planet and eliminating pests (i.e. humans) with poison. To Orson Scott Card's credit, the audio opera Ender's Game Alive incorporates these retcons and adds several other nuances (such as Formic queens not always being in direct control over all Formics) that weren't present before. The second set of spin-off novels also seems to retcon the state of the International Fleet after the end of the Third Invasion. The original novel implies that the IF was disbanded, largely due to the various nation-states remembering old grievances and ending their alliance. The Fleet School series claims that IF still exists but has taken steps to stay away from Earth-bound politics, seeking to ensure humanity's survival in case of another alien threat (or the possibility that not all Formic queens were killed; to be fair, that was a stupid assumption in the original book).
The first book of the GONE series has Michael Grant (author) toy with giving Astrid a power, in which she is stated to be a quasi-powerful freak with the power to tell if someone is "special" or "Important". As of the third book however, Astrid is explicitely stated to have No powers and her mutation is never mentioned again.
Michael Grant has stated in three interviews that he regrets putting Astrid's power in as A) he didn't think her power was interesting, useful or original and B) he wanted some of the main characters not to have powers to break the status quo a little.
Fans are still debating whether he was right to cut the mutation out however, with some fans arguing that her power didn't add much to the plot and that Astrid is already described as being beautiful and a genius. Other members of the fandom however argue that it's unrealistic (due to genetics and such) for her brother to be the most powerful freak in the series and for her not to have a power at all. They also think her power should have been kept in to avoid plot holes, and well, a retcon.
In the Guardians of Ga'Hoole series, different books have drastically incompatible images of Ifghar. In the Guidebook, he's a victim of bullies who was briefly attracted to Lil but grew out of it and wished her happiness in her new relationship. In the other books, he's a bullying Yandere who tried to rape her.
In Harry Potter and the Philosopher's Stone, Dumbledore flies to London for a supposed meeting, then flies back, arriving just in time to save Harry from Quirrellmort. We were probably supposed to believe that he was on broomstick. But later books introduce the ideas that wizards have instantaneous methods of travel: Floo Powder, Apparition, etc. Book 5 tells us Dumbledore went on a flying horse (Thestral) because he wanted to arrive late because the Ministry (who were supposed to have called the meeting) were annoying him. But couldn't he have just taken Floo Powder at a later time?
In the first book Rowling had Voldemort tell Harry that his father "put up a courageous fight" before he died, and there was no reason for him to lie to Harry at this point (although it's not inconceivable that he was exaggerating). Seven books on, James Potter is killed in flashback without raising so much as a finger against Voldemort, though James did try to hold him off, but was killed quickly, having forgotten his wand.
In an otherwise throwaway line in Quidditch Through the Ages, powerful wizards are said to be able to Apparate across continents. Come Deathly Hallows, however, this is quietly retconned; Apparition is now said to have a range of effectiveness less than that of Western Europe, and even Voldemort is forced to fly "within range" of a certain location before he can Apparate.
In all the books prior to the the fourth, the only immobilizing spells are Petrificus Totalus and a spell to summon chains, the former being temporary and the latter allowing an Animagus to escape. Goblet of Fire introduces the Stunning Spell, which paralyzes the user perpetually until the caster dies or the victim receives the counter-curse. While the kids couldn't have known the spell in the prior books, in some of the cases they were around adults who should've known the Stunning Spell and made a lot of situations easier.
The first three books state that Gryffindor last won the Quidditch Cup many years ago when Charlie Weasley was captain. This implies that Charlie had left Hogwarts a year before Percy even arrived. JK Rowling later stated that Charlie would've only graduated the year before Harry and Ron began their education. This makes little sense as it is implied that the last win was when Charlie was captain.
No two versions of The Hitchhiker's Guide to the Galaxy (radio series, books, TV series, movie, etc.) are exactly the same. And contrary to many fans misconceptions, the books are a retcon of the original radio series. Douglas Adams stated that this was intentional and there is no one canonical version of the story.
Later books (and short stories, although they are set earlier in the timeline) reveal that Honor is the result of genetic modification that gives her enhanced physical abilities and an increased metabolism. It raises issues about incidents in the early books she's described as accomplishing because of practice, training, and being raised on a higher-gravity world, instead of being inherently stronger and faster.
In the first book, when the story of Pavel Young's attempted rape and its aftermath is mentioned, the resulting stomping of said aforementioned rapist-wannabe is given as due to her martial arts training and world of origin. In Field of Dishonor, the winning of the duel is implied to be due to her rage and relentless practice. It's only well into later books that it's revealed she had a tad more of an edge than just the learned skills.
How prolong treatment works changed from when it was first mentioned in the second book of the series, The Honor of the Queen. It's explicitly noted that third generation prolong extends all stages of human lifespan, and a Grayson officer (Grayson not having that biotech at the time) is disturbed at visiting a Manticoran ship for the first time and seeing that the ship looks like it's being operated by children, as most of the younger personnel, chronologically in their 20s or early 30s in standard years, look like young teenagers, at most. Later in the series it's also mentioned that consensual sex on naval ships in the Manticoran navy is accepted (within reason). At some point, it was realized what the combination of those two facts unintentionally implied, not to mention the absurdity of combat troops like Marines being that physically immature. Later works in the series make a point to mention an additional treatment prolong recipients receive that neutralizes the effect until one has reached physical maturity, thus aging at a normal rate so a 20-year-old looks like a 20-year-old and not 12.
Len Deighton's Hook Line & Sinker trilogy retconned the events of his earlier Game, Set & Match trilogy. In the latter trilogy, Bernie Samson discovered that his wife was a deep cover agent for the Russians. The former trilogy changed this so that his wife was actually working as a deep cover agent for the British with her defection as a bluff.
In the Jurassic Park novel, Ian Malcolm is very definitely dead. They're even trying to get through diplomatic red tape to get him buried. By the time of The Lost World, Malcolm is alive and well, having only been... mostly dead.
Land of Oz: The Wizard was retconned within the first three books due to complaints. In the second book The Marvelous Land of Oz, it's mentioned that the Wizard stole Oz's rightful heir and gave her to an abusive witch to raise so that he could take the throne. The very next book, Ozma of Oz, changed it so that he wasn't involved in Ozma's kidnapping and didn't take the throne by force. He's shown to have never met Ozma before.
The Malazan Book of the Fallen underwent significant changes in its backstory between when Gardens of the Moon was completed and the initial publication of the Malazan series almost a decade later. Especially where Steven Erikson dealt with matters relating to the Fall of the Emperor, his rise as Shadowthrone, Dancer and his rise as Shadowthrone's sidekick Cotillion, the Patron God of Assassins, and their desire for revenge on Laseen for usurping the throne of the Malazan Empire. As a result, fans of the series have a term for retcons relating to backstory discrepancies between Gardens of the Moon (short GotM) and the rest of the Malazan series: GotMism.
In Stephen King's Misery, psychotic nurse Annie Wilkes forces Paul Sheldon to retcon the death of Misery Chastain in his series of romance novels. His first attempt is to simply Rewrite it, but he is forced to do a Revision in which Misery was buried alive in a coma when Annie considers the rewrite to be cheating.
In-verse example: Nineteen Eighty-Four is hugely based off this trope. Big Brother can rewrite history at will, and the masses have to eat it up. Retconning is done at the Ministry of Truth, the protagonist's main place of work, and mainly consists of editing out people who fell from Big Brother's favor and were "vaporized".
In The Odyssey, there are two sirens. Yet there is art that depicts three. How to fix, how to fix obviously, one of them committed suicide in a rage after Jason and the Argonauts! And another one after Odysseus got away! (Which raises the question of how the third one died... they never did settle that.) Older Than Feudalism.
The waterfall that separates the first and second precincts of Death is described as "getting louder" when an entity passes through it from the second to the first precinct in Sabriel, but that is retconned in Lirael where an entity passing through causes the waterfall to go silent, and thus agreeing more with the idea that a pathway must be made through the waterfall by magic.
A very confusing retcon occurs in regards to "Gore crows", bodies of dead birds reanimated with a single spirit. They are at first described as extremely dangerous due to many of the birds being able to overwhelm an individual, but later when Prince Sam kills one with his slingshot, the Disreputable Dog claims that the other gore crow, which took turns spying on the main party, would be killed because they are both linked to the same spirit. But if this were true, then the first description would not make sense, as soon as the individual was able to kill one of the crows, the rest would perish.
The Secrets of the Immortal Nicholas Flamel: In book 1, The Alchemyst, Scatty talks about fighting with Joan of Arc and is described as sadly telling her not to go to Paris, implying she's regretful of Joan's death. The very next book reveals that Joan is alive and well (and immortal to boot) and that it was Scatty who saved her from her execution making the aforementioned line rather weird. In a series that is otherwise very good with its continuity.
A Series of Unfortunate Events: So heavy that a number of companion books had to be written to fully explain them; these were themselves retconned. Handler originally thought the series would only last a few books, not the intended 13, and hence the first four books were essentially unconnected; V.F.D. was created as an ongoing plotline when it became clear the series could run 13 books, and details from the first four books were retconned to be part of the V.F.D. backstory to bring the entire series together.
In the Sherlock Holmes stories, Arthur Conan Doyle originally intended Holmes to die in "The Final Problem" (as the name kinda implies), but the Holmes fans were so furious (there's a story that an old lady screamed "murderer" at him on the street) that in the end Conan Doyle had to Ret Con his hero back to life in "The Adventure of the Empty House". Opinion pretty divided on whether this is a good thing or not: there are some very good Holmes stories after he comes back, but overall the quality does go down, due to Conan Doyle not really caring any more.
Several in the Star Trek Novel Verse. A particularly good example: in Star Trek: Vulcan's Heart, Romulus' capital was given the name Ki Baratan. It had previously been called Dartha, but that was in a story set a century prior. Later novels used the time gap for a reasonable Retcon: the capital's name changes as new regimes come to power. Now, books set in the 22nd or 23rd centuries use "Dartha", those set in the 24th use "Ki Baratan". The name change is explicitly mentioned in the first Star Trek: Titan novel. Another good example is the Andorian issue. The Andorians were initially portrayed somewhat differently between Star Trek: Enterprise and the Star Trek: Deep Space Nine Relaunch novels, but later books skillfully resolved the seeming contradictions. The various portrayals now add up neatly. Also, in the Star Trek: Enterprise Relaunch, the names of Xindi characters are a blend of screen names and those given in early novelizations. For example, the Xindi known as Dolim was named "Guruk" in the first novelization, so in later books his full name is given as "Guruk Dolim".
In the Star Trek Shatnerverse novel The Return, V'ger is established as a prototype or inspiration for the Borg Collective. In other novels, this origin seems to be ignored.
In Star Wars Legends, the prophecy in The Glove of Darth Vader was retconned to be about Darth Millenial, the titular glove became a Mandalorian crushgaunt, and the Prophets of the Dark Side were revelaed to be false prophets.
The Sword of Truth series: Presumably to explain the inconsistencies in the timeline generated by the official version of the Magic of Orden, the final book reveals that the Book of Counted Shadows, supposedly the instruction manual to safely use the power of Orden, is a fake. So far, so neat and tidy, until you remember that the climax of the first book and a large chunk of the plot of the second book (and, indirectly, most of the rest of the series) rest on Richard awakening his Gift by using it to kill his father - by tricking him into opening the wrong box - which doesn't apply if there never was a right box, meaning Richard's Gift should not have awakened. Not that the logic was entirely convincing to begin with...
Played for Laughs in To Be or Not To Be: That Is the Adventure, where if you choose the crazy options when interacting with Ophelia early on, the narrator gets annoyed and retcons an earlier scene to have Hamlet tell Horatio that he's going to act crazy for a little while in an attempt to find a justification for your actions.
The Hobbit was written well before J. R. R. Tolkien came up with the plot for The Lord of the Rings. In the original story, Gollum's wager for the Riddle Game was a "present", which turned out to be the ring. Once The Lord of the Rings existed, having Gollum ever do anything which might lead to him losing the Ring on purpose suddenly made no sense at all, and so that chapter had to be Ret Conned. Tolkien dealt with it by writing a revised edition and during The Lord of the Rings later making Bilbo an Unreliable Narrator and having Gandalf shake the "true" story out of him.
Mention of "cheese and tomatoes" is changed to "cheese and pickles" in later editions because tomatoes were from the Americas, and wouldn't exist in Middle-Earth. Note 'tobacco' is referred to as 'pipeweed' or 'baccy' and potatoes are called 'taters'.
The character of Elrond is also slightly different from one book to the other: when Tolkien wrote The Hobbit, he gave him the name of Elrond, which he had previously used in drafts of The Silmarillion for the son of Eärendil, but did not make him the same character, which is why it isn't clear in The Hobbit whether Elrond is actually counted among Elves - he appears to be a Man. It was only when writing The Lord of the Rings that Tolkien explicitly made the character that appeared in The Hobbit, and the character that appeared in The Silmarillion, one and the same.
Though Tolkien had been working on the myths of Middle-earth for a long time, The Hobbit was not originally set there when it was written, so the earliest editions contain things that were later eliminated, like references to policemen.
If you're interested in studying Tolkien's languages, the many changes and revisions in the linguistic history across Tolkien's life can be a nightmare. Many of these changes are "internal" (not in texts intended for publication, so not proper retcons), but a lot aren't. In fact, most "corrections" to the second edition of LOTR were linguistic in nature. Other changes involved names that weren't updated as the languages changed: the name Fëanor initially was pure Sindarinnote Called Noldorin back then. for "spirit of fire"; but decades after, the pure Sindarin form would have been Faenor, and the name had to be explained as a mix of Quenya and Sindarin.
The Vampire Chronicles: The Lestat that appears in Interview With the Vampire is a rather stupid, petty villain with a streak in banal evil. Anne Rice wrote in her later books that this portrayal was merely Louis' spiteful gossip and/or misunderstandings. If The Vampire Lestat is read before Interview With the Vampire, the impression that Louis is giving the account as a bitter ex-love becomes pretty unmistakable.
A plot point in one of the prequel novellas is Spottedleaf wanting to be a warrior. In the main novels (written several years beforehand) she is nothing but enthusiastic about her job as a medicine cat, her entry in the official guide never mentions such aspirations, and her personality is distinctly pacifist.
The Dark Forest always existed, and was always entering cats' dreams; it's just that nobody (not even inhabitants of the Forest's heavenly analogue, which is mentioned constantly in all the books) ever mentioned it until series 4. Even though it leaves actual physical wounds on the bodies of cats who fight in it.
SkyClan belong in the canyon, in part because it's close to the Twolegplace many SkyClan members still have ties to. Except they don't, and someone has to bring them to the lake once ShadowClan's fall conveniently leaves an empty space.
The series originally took place in Britain and the original forest was based on an actual forest. Since the second arc, more and more animals that shouldn't be there have popped up. The series currently takes place in an undefined country.
Leafpool has a prophetic vision in which she sees that Squirrelflight and Brambleclaw will be together until their deaths. But a series later, he breaks up with her because she lied about being their kits' mother.
Somewhat similarly, the Xanth series began addressing early-book continuity issues by saying the magic-dust madness area was spreading and screwing with reality.