Also, Harry fails to notice the Thestrals pulling the carriages at the end of Goblet of Fire only to do so the next book. J.K. Rowling herself says that this is because she didn't want to add the Thestrals at the end of a book, stuffing it. Her in-universe explanation is that takes some time to "process" a death and come to terms with it; only after then can you see Thestrals. It is also worth noting that he didn't see his parents die at all, not even his mother. He was lying in the crib and only saw green light fill the room. Both J.K. Rowling herself and the seventh book confirm this. This explains why he was not able to see the Thestrals from the beginning of the first book. Link.
It still creates a major plot hole for the films. In the novel, he passes out before he can see Quirrell burn up, but in the movie, he's a first-hand witness to the whole process in all its nightmare-inducing detail. Maybe Quirrell's death wasn't sufficient since he only had a "half-life"? It also creates a plot hole for the movies in that Harry sees his mother die, or at the very least, Snape holding her body, though the explanation that the death needs to be "processed" might explain this.
In the first book, Voldemort tells Harry that his father "put up a courageous fight" before he died, and had no reason to be lying to Harry at this point. Seven books on, we see this death in flashback: though James did try to hold Voldemort off, he was killed very quickly, having forgotten his wand. Perhaps, given that he was going against the Evil Overlord unarmed, instead of begging for mercy, Voldermort would have considered fighting him at all to be brave. He didn't say he was a tough fighter, just a brave one.
In Harry Potter and the Goblet of Fire, Harry and Voldemort duel, and because their wands are made from the same source (Fawkes the Phoenix), it creates a Phlebotinum effect that causes the spells Voldemort cast to pop back out of his wand in reverse order. An early edition of the book suggested that Harry's mother was killed before his father, which contradicted every description of their final moments. Rowling later admitted the mistake. Later versions of the book, and the movie, fix this.
The Death Eater called Rookwood has the first name Algernon in early editions of the book he's first referred to in, Goblet of Fire; it changes to Augustus in Order of the Phoenix. Later editions of Goblet have corrected his name to Augustus.
Sirius stated that Bellatrix Lestrange was a friend of Severus Snape's during their Hogwarts years, but according to the Black Family Tree Bellatrix is about 9 years older than both Sirius and Snape and would've graduated from Hogwarts before they even started.
The Black Family Tree however has many other errors such as both Bellatrix's father and grandfather having been only thirteen when they had their children, Araminta Meliflua's exclusion (Sirius points to her name in the book), and Sirius's "aunt" Elladora being born many generations before (despite his dialogue suggesting that he knew her).
Charlie Weasley was said to have been a legendary seeker on the Gryffindor Quidditch Team with skills comparable to Harry. Many characters allude to the fact that Gryffindor hasn't won in 7-8 years which implies that he hasn't been in school for that long. JK Rowling has since announced that Charlie had only just left Hogwarts the year before Harry and Ron started, however this means that Charlie would've only won the first year he played Quidditch (assuming he started playing during his second year) which doesn't really live up to his reputation as a "legendary seeker".
At one point Molly Weasley mentions the previous gamekeeper before Hagrid, a character named Ogg, while reminiscing about her time at Hogwarts. This becomes a problem when we learn that Hagrid became the gamekeeper immediately after he was expelled in the early 1940s which was before Molly's time. It's possible however that Hagrid was not the only gamekeeper and became an assistant or apprentice gamekeeper under Ogg. He was only 13 when he was expelled after all.
Rowling at one point said that Dumbledore was around 150 and McGonagall was roughly 70, however she's since said that Dumbledore was born in 1881 (making him 115-116 at death) and McGonagall was born in the 1930s.
The Pottermore website is supposed to give information that was All There in the Manual, but some of it is hard to reconcile with the books. For example, the site says that Voldemort possessed Quirrell as soon as they met...but Quirrell is able to shake hands with Harry in Diagon Alley (where he isn't wearing his turban), and his dialogue implies that Voldemort starting Sharing a Body with him after the failed Gringotts break-in (after which he is wearing the turban).
Similarly, in the Distant Epilogue (set in 2017), James Sirius sees his godbrother Teddy and cousin Victoire kissing and is completely shocked. Pottermore has Rita Skeeter articles set in 2014 that characterizes them as Make-Out Kids. So apparently the two were not only dating, but making headlines for three years before James had any idea that the two were an item? Possibly justified by Rita being a known liar, but even then her accuracy and James' obliviousness is a bit odd.
In The Bartimaeus Trilogy, it was established that certain commoner children are whisked from their homes to be apprenticed to a magician, and forget their family and even their name. Yet Arthur Underwood has a desk that's "a family heirloom", and a photograph of his grandmother in his study. Also, in The Amulet of Samarkand, Arthur says Bartimaeus would have to be a djinni of "at least the thirteenth level", yet later he is described as fourth-level. And later as sixth level. On the other hand, Arthur's magical abilities are held in contempt by pretty much everyone (including his twelve-year-old apprentice), so that's not quite as bad (and depending on where the information on Bartimaeus' levels came from, spirits do enjoy inflating their own reputations and deflating their rivals').
Tom Holt's J.W. Wells & Co. series. The first book, The Portable Door, stated that using the Door for a journey of more than an hour was almost impossible, and using it for any real length of time was exhausting. By The Better Mousetrap, Frank Carpenter is living his entire life about two decades in his past thanks to the Door, without really experiencing any negative effects whatsoever.
Dr. John Watson of Sherlock Holmes seems to get caught in these rather often.
Watson served as an Assistant Surgeon of the Army Medical Department (attached to the 66th Foot) in Afghanistan, but was discharged following an injury received in the line of duty during the Battle of Maiwand. In a Series Continuity Error, his wound shifted from shoulder to leg. This is referenced in the Guy Ritchie film, where Watson limps throughout the film but is also wounded in his shoulder by shrapnel near the end, and in Sherlock, where he was shot in the shoulder, but has psychosomatic pain in his leg. Carole Nelson Douglas used this error to underpin the plot of her Irene Adler novel A Soul of Steel, in which Watson had to be saved from an assassin without informing him; according to her book, Watson received both injuries, getting the second wound in hospital while delirious from a fever, so he did not recall getting it.
Doyle's stories — which sometimes assigned Watson a wife, sometimes didn't, and sometimes referred to Watson as a widower, never in chronology — hopelessly confused the issue of just how many wives Watson had. W.S. Baring-Gould, author of a well-researched "biography" of Holmes, decided Watson had three wives. Others have suggested numbers up to six.
Both "The Final Problem" and its prequel, The Valley of Fear, involve Watson hearing about Moriarty for the first time. The latter apparently makes Moriarty aware of Holmes' activities against him two years earlier than the former does.
To add insult to floating injury, Watson's first name is in question. It's stated as John H. Watson at least once, but in "The Man With the Twisted Lip" his wife calls him James. Scholars (and the BBC series) have tried to get around this by expanding Watson's middle initial H to 'Hamish', the Scottish equivalent.
Professor James Moriarty seems to have a brother, Colonel James Moriarty. Doyle has a James problem.
In "The Adventure of the Blanched Soldier", Holmes refers to "the case which my friend Watson has described as that of the Abbey School, in which the Duke of Greyminster was so deeply involved". This seems to be intended as a reference to "The Adventure of the Priory School", but misremembering both what religious building the school was named after, and the title of the Duke of Holdernesse. (The Watsonian explanation for this is that Watson was using pseudonyms to avoid embarrassing a member of the nobility.)
Christopher Paolini, of Inheritance Cycle fame, gave Murtagh dark brown hair in Eragon, but his Adaptation Dye-Job in the movie seems to have confused the author enough for Murtagh to have a different hair color by Brisingr.
Earlier Andrew VachssBurke books said that badass martial artist Max the Silent was a Mongolian, but the first mention of his nationality in Terminal has him as Tibetan. It's even more messed up when the next time says he's Mongolian again. Supposedly the Tibetan thing is established in the first book, but as later books never spell it out clearly in full, you can see how this confusion arises.
In World Engine, the Necrons crumble to the ground when killed. This contradicts every single other piece of Warhammer 40,000's expanded universe, where the Necrons either self-destruct or are teleported out.
Sancho talks about using a sword at Part I Chapter XV and the Barber mentions Sancho has a sword in his hips at Part I Chapter XLVI, but at Part II Chapter XIV, Sancho denies ever having a sword.
The name of Sancho's wife changes at the same page in Part I Chapter 7 (Juana Gutiérrez and Mari Gutiérrez), and the same at Part II Chapter V (Teresa Panza and Teresa Cascajo) and in Part II Chapter L, Teresa Panza).
In the first book, Visser Three snidely comments that it's an honor to meet Elfangor right before killing him. This directly contradicts the backstory concocted later on, wherein the two characters have extensive history. This can't even be attributed to the weird timeline/memory changes that happen near the end of that backstory, since it's alluded to in Visser.
Animorphs has quite a few of these in the first few books, with one of the more obvious being Jake using thought-speak on Tobias when Tobias is a cat and Jake is still unmorphed. Later books have them discuss the inability to use Thought-speak outside of morph. They're known among fans as Katherine Applegate Screw Ups or KASU's.
Book 34, "The Prophecy," features Aldrea from "The Hork-Bajir Chronicles" returning due to her personality being recorded before her death. Trouble is, she's consistently referred to in the book as having voluntarily trapped herself in Hork-Bajir morph, when "The Hork-Bajir Chronicles" made clear that it was accidental as she was knocked out past the two hour time limit. Hell, the book can't even keep continuity within itself, as Aldrea announces right away that she doesn't know where the MacGuffin du jour is, but when she brings this up later, everyone acts like they're hearing it for the first time.
The Dirk Pitt Adventures novels by Clive Cussler all take place in the same continuity, with past events often mentioned. At the end of Trojan Odyssey, Dirk marries congresswoman Loren Smith, his love interest for most of the series. In the preparations for the wedding, it states the Loren's parents flew out to attend. The plot of Vixen 03 revolves around the murder of Loren's father twenty years previously, and it was stated that he was already a widower when he died.
In The Chronicles of Narnia, Aslan tells people on various occasions that "no one is ever told what 'would have happened'". The wording is unequivocal. Then in The Magician's Nephew he tells someone "what would have happened". He even uses those very words. It may be writer error. On the other hand, he created Narnia, has used deception before (in The Horse and His Boy, he deliberately tricks Aravis), is "not a tame lion" (he doesn't usually follow any rules but his own), and the decision in question was one of the most important ones in Narnia's history.
V. C. Andrews' series tend to have this. There are various reasons for this, but one main one is that VC Andrews had become ill and died before several of her series can be completed, and a ghostwriter completed the series. An example is in Flowers in the Attic, by Andrews, numerous things are referenced or mentioned by Corrine, only to be either retconned or changed in the prequel Garden of Shadows, which was written by a ghostwriter.
The ghostwriter has done this in his own series. For instance, in the Landry Series, Ruby's mother is named as Gabrielle in the first three books and Gabriel (a masculine name) in the last two. Her account of her own death also differs from the description that Grandmere Catherine gave.
In the Flashman series, Flashman has an uncle Bindley who works at an office responsible for assigning military commissions, and at a couple of points, Flashman is forced to seek his help getting a commission. Consistent in the books is that Bindley dislikes Flashman, but the reason for this depends on the book. In the original novel, Bindley is from the aristocratic side of the family and doesn't like Flashman because he's from the commoner side. Later books reverse this by having Bindley as the commoner and resenting Flashman, who looks down on him. It's possible that this is a deliberate use of Unreliable Narrator, since the novels are supposedly extracts from memoirs Flashman wrote as a very old man.
Children of the Red King: When Lysander's mother appears in book three, she's named Jessimine. In book five, she's named Hortense, which was the name of one of her daughters.
Happened towards the end of the Teenage Worrier books. Among other things, Hazel's name in the first book is Hazel Williams and she attends an exclusive school called St Mary's Academy; in the last, her name is Hazel Appleby and her school is St Cheyngangg's (with no mention of her changing schools).
In Mercedes Lackey's Dragon Jousters cycle, the only one of Kiron's sisters whose name is given is the one who was so severely beaten by soldiers in the backstory as to cause permanent brain damage. However, at one point in the first book, Kiron remembers her name as Deshara; barely a page later, her name seems to be Dershela. And in the fourth book, when we finally meet Kiron's missing-presumed-dead mother and one of her daughters (and the apparent brain damage strongly implies that it's the same daughter), her name is Iris.
There are also a large number of dragon riders Kiron trains starting in the second book. Most aren't given much in the way of personality. In successive books those personalities are not kept straight, with Gan taking on Oset-re's traits and Oset-re not having any himself.
In the first book dragons are noted to eagerly devour any scorpions they find, seeing them as special treats. In the last book they are too small for dragons to notice, so a scene with a flood of scorpions has two dragons ignoring the creatures entirely instead of licking them all up.
The chronology of the Tedrel Wars and Selenay's early reign is set in Exile's Honor and Exile's Valor. Skif's teacher in Take A Thief claims to have been crippled at the beginning of those wars, but according to him they started at least 15 years before the dates in the "Exile's" books.
The first two novels of the Mage Storms trilogy cover less than a year of time. Tremaine manages to age from around 30 years old at the beginning of "Storm Warning" to 45 years old in "Storm Rising". Another inconsistency in those novels is the official cult of the Eastern Empire, which goes from the "Forty Little Gods" to the "Hundred Little Gods".
The timeframe in which Selenay's first husband and father-in-law die changes continuously. When the series was first written, it was implied that the two died at roughly the same time with the notifications of their deaths crossing en route. In By The Sword, it was stated that learning about Thanel's attempted assassination of Selenay was a contributing factor to his father's decline and death. In Exile's Valor, Thanel's father dies about a year before Thanel himself does.
The Elemental Masters novel The Serpent's Shadow cannot make up its mind about the name of Maya's father. He is mentioned by name twice. The first time his name is Nigel, the second time his name is Roger.
In "Reserved for The Cat", Ninette's mother is named Marie or Maria almost interchangeably. Marie would be more likely for a Frenchwoman, but...
And in Unnatural Issue, Susanna makes a charm bundle and uses it to create a doppleganger of herself so she can sneak off to practice magic. When she runs away from home she's specifically described as burning the bundle and scattering the ashes, as it could be used against her if it got into her evil father's hands. Yet towards the end of the novel it's said that the bundle could not be destroyed by mere burning as it was a magical object and Susanna still had it with her—conveniently, as the good guys could then use it in their plan to draw out dear old dad.
Happens many, many times with the Warrior Cats seriesnote A full list of inconsistencies and typos can be seen here: characters often change pelt colors and occasionally flip genders, sometimes they'll forget what certain characters know and don't know, time passage will be inaccurate, and some details about Clan life and the history of the Clans have gotten changed around. Seeing as there's dozens of books and side book, over a thousand characters, and multiple people writing the series, it's only natural that things get mixed up once in a while.
Happens occasionally in The Dresden Files, usually minor instances, which author Jim Butcher tends to acknowledge on his website. In Proven Guilty, the narration describe's a family's minivan being crushed flat, with the next chapter mentioning the whole family piling into the same minivan. In Ghost Story, a gunman's weapon changes from a snubnosed revolver to a semiautomatic between chapters. In Grave Peril, the name of the woman Bianca killed in Storm Front changes back and forth between Paula and Rachel.
There's a few in the Discworld novels. A lot of the continuity errors are explained retroactively in Thief of Time — the History Monks have had to repatch history several times, and it tends to have some effect on the present as well. PTerry said in interviews, "There are no continuity errors, only alternate pasts."
Apart from the characterisation of Death and the Patrician, there's things like the gates of the Assassins' Guild which in Pyramids never close because Death is always open for business (and also because they rusted years ago) but are firmly closed at the end of Men at Arms.
The confusion in the early books about who trained the young Esme Weatherwax is eventually resolved when she explains she went round all the local witches and basically bullied them into teaching her.
It was also explained in a later book that Anthropomorphic Personifications take on human characteristics if they spend too much time around people; Death, what with keeping Ysabelle and Albert around so much, and studying humans actively, was certainly exposed to them and so changed and grew in personality.
In a clear case of the writer and editor falling asleep, in at least the original hardcover of A Song of Ice and Fire's A Feast for Crows, Princess Arianne discusses the implications of Lord Tywin's death and Cersei coming to rule. A scant three pages later, she is first informed about it happening.
Renly is first described as having green eyes, despite the fact that all Baratheons having blue eyes is a plot point (and his eyes are described as blue for the rest of the series)
In the Garrett, P.I. series, Glen Cook once had his sleuth refer to his stable-keeping friend Playmate as "Sweetheart". Cook later justified this, having Garrett mention how he'd once made it a practice to call Playmate by an alias, to divert the attention of some thugs who might make trouble for his friends.
The Vampire Chronicles is riddled with continuity errors, some of which can be written off due to the fact that different books have different narrators. However, it's hard to understand how Lestat's eye-colour, even self-described, keeps shifting.
In "Queen of the Damned," Jesse Reeves sees spirits as a human but completely loses this ability when she becomes a vampire. It's made quite clear that vampires do not possess this ability. In the next book, Claudia's ghost appears to Lestat... well, it's a ghost of a vampire so maybe the rules are different. However, Merrick becomes a vampire and retains her ghost-seeing abilities, which nobody seems to think is that unusual.
In Interview, Louis mentions that Lestat received help from another vampire by the same maker. Future novels show that he was the only one turned by Magnus. He also receives help from another vampire in New Orleans, though he makes no mention of him in his own novel. These may be explainable by the changing narrators.
The Elric Saga: Elric's mother died giving birth to him, according to Elric of Melniboné. However, in The Sailor on the Seas of Fate he reminisces about his parents in a way that suggests that his mother as well as his father was still alive at a time when he was old enough to remember her.
In the Spellsinger series, Mudge the otter has kids as of the sixth book. How many, and what their names and genders are, changes from #6 (daughter Prickett and two sons) to #7 (son Squill, daughter Neena), and again to #8 (Squill and Nocter, genders not stated). Granted, Mudge isn't the most organized of fellows, but you'd think he'd keep track of who his kids are.
The story "Bertie Changes His Mind" (reprinted in Carry On, Jeeves) revolved around the fact that Bertie was considering moving in with his sister and her family, although they never actually appeared. Later, in Thank You, Jeeves, Bertie is asked whether he has any sisters and replies in the negative.
Another example from the novels: an early novel centers around Mr. Stoker and his daughter Pauline. When Bertie asks Pop Stoker which daughter he's referring to in the course of conversation, Stoker replies, "I have only one daughter." A few books later we're introduced to a second daughter, Emerald Stoker. But then, one must not overthink these books.
The Riftwar Cycle: The later novels have quite a few continuity errors. For instance:
Eric von Darkmoor mentioning that he had never married when he got married in Wrath of a Demon King.
The Tsurani Emperor appointing a Warlord in Wrath of a Mad God, generations after the title was abolished and replaced with a similar position with no attached political authority in Servant of the Empire.
In the same book, mention of House Minwanabi, every member of which had committed ritual suicide at about the same time as the position of Warlord was abolished.
The story Jimmy the Hand taking place in Land's End, resulting in virtually all of the nobility of that small region dying, which makes the long-established recurring minor character Squire Locklear of Land's End's backstory nonsensical, especially since his first appearance in the timeline is only about a year later, nowhere near enough time for the new Baron, who had no siblings, to marry, produce an heir, and have that heir grow old enough to be sent as a squire to the Prince's court.
The Book of the Dead can't seem to decide what colour Constance's eyes are; at the beginning of the book, they're violet, at the end of the book, they're blue, and in the preview for the next book, they're "dark".
In Aunt Dimity's Death, Bill tells Lori that his mother was struck and killed by a bus when he was twelve, even giving this as the reason his father avoids public transport. In Aunt Dimity and the Village Witch, Jane Willis is said to have died after a long bout of cancer. Unlike the backstory of Derek Harris and his fraught relationship with his father, there is no secret that Willis Sr. kept from his son; the author readily admits this was a mistake on her part.
Piers Anthony wrote Geis of the Gargoyle partially to explain and straighten out continuity errors that had crept into the chronology of Xanth.
Stephen King made a minor error in Cujo. The father, Joe Cambers, is described as having blue eyes on one page, and brown eyes on the next. There's no supernatural explanation for this, it's just an error.
The Well World series by Jack Chalker had a fair number. Some of these are explained by there being a very long time-gap between the early books and the later ones, so the author merely forgot the prior details.
Several of the books contain maps of sections of the Well World, and a careful look at them reveals that some hexes are named differently or in different places in different books.
And then there is the case of the hex that seems to contain 3 completely different main species. In one book, they are quadripedal lizards with excellent natural camouflage abilities. In another, they are bipedal magic-using. And in a third they are snake-like men with semi-invisibility.
And also a race that, despite having a major character being a member of the species, and their warlike battle lizard traits being a fairly large background plot point, by a later book, they are man-sized mosquito-like things.
In early novel Moonraker, Bond is eight years away from compulsory retirement from "00" Section at 45. Subsequent novels certainly take place over a period of more than eight years.
Additionally, in Casino Royale Bond recalls facing off against enemy agents over a gaming table before World War II. However, his obituary in You Only Live Twice indicates that he joined the secret service after leaving the Navy in World War II, and that he enlisted at the age of 17.
The Outcast Dead states that Magnus the Red warned the Emperor about Horus's treachery after the massacre at Isstvan V, contradicting every other book that put Magnus's warning coming before Horus's opening move at Isstvan III.
In A Thousand Sons, Ahriman and Magnus tell Lemuel Gaumon about the history of the Thousand Sons, including how other primarchs like Mortarion and Corax looked down on their psychic abilities before Magnus was discovered. Deliverance Lost shows that Corax was the second-last primarch discovered (Alpharius has always been the last one), so he couldn't have been around to voice his dislike of psychics.
In Protector of the Small's third book, Squire, Kel seems to jump from age fourteen to sixteen. It's also stated that her first tilt against Raoul was her first tilt against a live person when she'd already done that in Page (Pierce said she simply forgot). In Lady Knight, Raoul says that nobody has ever been allowed in the Chamber of the Ordeal twice—except that it's a part of the coronation so Jon the knight king did in the first quartet, and Raoul is close enough to Jon to know that.
Horatio Hornblower has permanent powder burns on his hands from his old adventure in capturing the Castilla. The details of this seem to change every time the story is told. Even which hand is burned is inconsistent. In Beat to Quarters, he mentions his right hand was burned during the Castilla's capture while he was a lieutenant, yet in the short story "The Hand of Destiny", which details the capture while he's a lieutenant, it's his left hand that's burned, and Hornblower and the Atropos, in which he's a post captain in command of the 20 gun Atropos (a post ship, the smallest ship to warrant a post of full captain), he aids in the capture of the Castilla in a completely different action from what's described in "The Hand of Destiny", and doesn't receive a powder burn to his hand.
There are quite a few in the Dragonriders of Pern series, sometimes within the same book - for example, several minor characters get their names changed between books, and T'bor's dragon gets referred to as both Orth and Piyanth in Dragonflight.
It's been suggested that the fact Liessa Wyrmbidder suddenly becomes Lianna in her last mention during the Pern parody section of The Colour of Magic is an intentional nod to the above. According to Sir Terry Pratchett it was a typo that crept in during the printing process.
In the short story collection Short Trips: Time Signature, there's a bit of confusion about the Doctor's companion Issac/William. He's first introduced in "An Overture Too Early" by Simon Gurrier (the book's editor) as Isaac, a defector from the USSR during the UNIT era, who claims to be a past companion of the Doctor - although the Third Doctor realises that this is in his future. He later appears in "Fishing Trip" by Ben Aaronovitch as William, a young man from present-day Slough who the Sixth Doctor takes fishing. This gets tied together in "The Earwig Archepelago" by Matthew Sweet, in which William takes the name Isaac when he gets involved in local politics in a 1950s Ruritania, at which time the Doctor loses track of him. But between these stories, in "Walking City Blues" by Joff Brown, William refers to "the men who run the beureaux back home", suggesting that Brown missed that William didn't originally come from Eastern Europe.
It's understandable, since the books must have been mostly written simultaneously, but in the Eighth Doctor Adventures novel Dreamstone Moon, it's mentioned Sam spent some time teaching herself to use 23rd century computers including the "bowlboards" that have replaced keyboards, and in the next book, Seeing I, she panics when she realises a 23rd century keyboard isn't QWERTY.
The third and fourth books of the Pit Dragon Chronicles were written 22 years apart. Many things that were important in the first three books are randomly changed in the fourth - such as a psychic connection between dragon and handler being taken for granted suddenly being a horrifying new idea, or dragons being Starfish Aliens mentally with mysterious degrees of intelligence suddenly just being animals.
For example, in the first book, Percy dreams with a girl he has never seen before, but somehow already knows who she is. In the second book, he has a dream with the same girl, but is neither able to identify her, nor notice that he already dreamed about her before. And since the entire series is narrated through his point of view, it would be impossible for him to forget that first dream.
The Chronicles of Amber wobbles back and forth on a number of things over the course of its 10 books. Most notably, which of the royal family are full-blood siblings (Corwin initially says he and Random are; this is then changed to Corwin and Eric) and how time works around the Courts of Chaos. It's established at first that a short time anywhere else is a long time in the Courts, allowing for there to be nearly-adult children of couplings that happened mere days earlier. Later when Corwin actually goes there, a subjective hour or so in the Courts turns into several days back in Amber.
Early in the series it is established that Moroi monarchs have the right to choose their own successor. The only acknowledged limitation to this right is that they can not choose a heir from their own royal clan. That is why Evil Prince Victor Dashkov considered himself the most likely heir to Queen Tatiana Ivashkov. Because she was likely to choose him, at least until his fatal disease discounted him as a choice. In the Succession Crisis of Last Sacrifice it is instead established that the Moroi political system is an Elective Monarchy. So monarchs have no right to choose their successors after all.
Tatiana was murdered before she had a chance to name her successor, which meant that they had to go with the back-up option of the election.
There is a minor one, involving the powers of Jill. When introduced in Shadow Kiss, Jill tells Rose that she is an air user. She is later established as a water user instead, even given some training by older water user Mia Rinaldi. According to Word of God the first instance was a typo, and Jill's element is water.
In the seventh book of the Maximum Ride series - Angel, Maya says to Max, "Gee, I haven't seen you since you tried to kill me." The most recent time they met, in the third book - Saving The World and Other Extreme Sports, they were having a civilized conversation.
When Piers Anthony re-introduced several Adepts in the second Apprentice Adept series, their powers had changed from their original appearances, while ostensibly being the same characters:
Adept Green changed from using gestures and hand signs to work his magic to being a fire mage. This seems to be based solely on a scene in Juxtaposition, where a wall of flame warns Stile and Lady Blue away from Green's territory.
Adept Yellow changed from being the potions mistress of the Adepts to being a Beastmaster. Capturing animals and selling them off to others was a side practice of Yellow's in the original (Which she did with the use of a potion that lured them into her territory).
Adept Tan's Evil Eye was first shown as being as magically versatile as any other Adept's talent, limited only by line of sight. In the second, Tan and his twin children possessed the Evil Eye as a means of just mind control.
In Gods, Nikita's mother is called Lilianna, while in The Girl, it's Irena. Admittedly, as an assassin, Irena likely has several names.
Ernest has only obtained his middle "e" sometime between Gods and The Girl.
In Gods, Nikita and Kosma have been described as twins, with Nikita being Kosma's Cool Big Sis and Irena/Lilianna taking both of them away when she ran from Ernest. In The Girl, they're half-siblings instead, with Kosma being older than Nikita (and Irena) by several hundred years. Their sibling dynamic is also reversed.
Early in The Tamuli Emperor Sarabian considers beheading his Prime Minister - and when Queen Ehlana of Elenia asks under what charge, replies "This is Tamuli. I dont need charges. I can have his head chopped off if I decide that I donít like his haircut." The rest of the series makes much of the fact Sarabian is a powerless figurehead who has no say in the government, and needs Ehlana to teach him how to be a "proper" absolute ruler.
The Heroes of Olympus has a minor one. In The Lost Hero, Piper's dad has an assistant named Jane. In House of Hades, her name was Jessica instead. Rick Riordan joked on Twitter that "Her name is Jessica Jane. JJ to her friends." Then he admitted that he just plain forgot.