JK Rowling developed a full and detailed world when she wrote the Harry Potter books. The lore that was included in the initial story felt so complete to audiences that many will analyze the events that surrounded Hogwarts and the wizarding world as if they were real-world history.
With such in-depth lore, Harry Potter fans are very passionate about adhering strictly to the canon story. However, there are some parts of the movies that fans have become so used to that they completely forgot that they were never in the books. Whether because they fit in with the story so well, or because they are repeated so often they just started to blend in, these details are very easy to mistake for Rowling's canon.
Dudley Falling Into The Snake Enclosure
In the Sorcerer's Stone book, Harry accompanies the Dursleys to the zoo for Dudley's birthday. When Dudley pushes him aside to get a better look at a Burmese python, Harry's temper gets the better of him and he accidentally makes the glass of the animal's enclosure disappear, causing Dudley to fall in and become trapped instead.
This scene is easy to consider canon because it is so similar to the book, with one difference: Dudley never falls into the enclosure. Instead, he is simply struck terrified as the python escapes and comes near him. Becoming trapped was a little more exciting and mysterious, and gave Petunia something to scream about.
Snape's Calm Demeanor
Alan Rickman's portrayal of Snape left such an impact that it's easy for fans to forget that his demeanor can be very different in the books. In the movies, Snape is cool and collected. He relishes in his students' unhappiness, but he always stands tall and speaks in a slow, calm voice.
However, the Snape in the books is even less likable than in the movies. On more than one occasion in the books, Snape breaks into childish fits of rage. His hate for James, Sirius, and Harry causes him to act out in blatantly immature ways, making it even harder for fans to accept Severus Snape's redemption arc.
The First Task
It's easy to see when comparing the Harry Potter books and movies that the films often favor drama. In the Goblet of Fire movie, the first Triwizard task goes very wrong when the dragon breaks free of its chain and chases Harry all around the castle. The scene showcased a lot of great CGI and created an exciting and easy-to-follow scene for audiences.
In the books, this addition would have made very little sense. Fans have criticized the fact that no one batted an eye when a dangerous dragon became loose. However, the drama of the movie scene made it easy to forget how the task actually went in the book, which simply involved Harry tricking the dragon into following him as high as it could go, and diving to retrieve the egg.
Priori Incantatum
Every Harry Potter fan knows what it looks like when two wizards duel in the movies. They will volley spells back and forth, until their spells collide, creating a link between their wands that can be used as a sort of reverse tug-of-war game. This is first seen between Harry and Voldemort in Goblet of Fire but is later repeated by several other strong wizards until it became an extremely common phenomenon.
This connection means something entirely different in the books. The only time that two dueling wands will connect is when their core comes from the same magical creature, which applies to Harry and Voldemort and is an example of how it was Harry's destiny to be victorious at the graveyard.
Harry's Feelings About Luna
Luna Lovegood is very strange, and one of the best Harry Potter side characters. In the movies, Harry accepts her weirdness without a second thought. He invites her to Slughorn's party in The Half-Blood Prince, saying that she is "cool." He also rarely bats an eye when she shares some of her and her father's outlandish beliefs.
However, if a fan has grown used to this dynamic, they might be a little surprised to reread the books and remember how Harry sometimes thought of her. He does care for her deeply, but when he invited her to the party, it was on an impulse. He regretted it immediately, dreading what she might choose to wear or say. He was only made to feel slightly better about his choice when Ginny praised him for it.
Cho's Betrayal
Cho and Harry's relationship was always doomed to fail. Harry had witnessed her boyfriend's murder, so they didn't get off to a good start. In the Order of the Phoenix movie, Harry stops talking to poor Cho when he learned that she had told Umbridge how to find Dumbledore's Army. Even after he learned that she had been given a truth potion, he never made an effort to reconnect.
Fans might forget that this was not how things went down in the book. Cho was very confused about their relationship, and Harry was completely clueless about what to do. When Cho became jealous of Hermione, their whole relationship just sort of fell apart - and of course, in the books, Cho did not spill the beans, her friend did.
Flying Death Eaters
Dark magic is appealing to some wizards because of the power that it can grant. In the movies, one of these powers is flight. It may be a visual representation of apparating and disapparating, but Dark Wizards are often seen soaring around in streams of black smoke, in both the Harry Potter and Fantastic Beasts movies.
In the books, there are complex kinds of magic that can allow a wizard to fly. The Order of the Phoenix was surprised to see that Voldemort could fly when he pursued Harry in Deathly Hallows, and Harry later sees that Voldemort passed this knowledge on to Snape. However, no other Death Eaters are ever described to be capable of flying.
Lily's Gift To Slughorn
When Harry uses Felix Felicis in Half-Blood Prince, Slughorn shares a touching story about once finding a fishbowl on his desk, that contained a flower petal that turned into a fish (which he named Francis). He told Harry that the fish had been a gift from Lily and that it had disappeared the day she had died. This story was so beautifully tragic, and fit Lily's character so well, that some fans completely forgot that it never happened.
Lily had been a favorite of Slughorn's, and Harry used that to convince him to hand over his memory of Tom Riddle, but Francis the fish has never been book canon. It's such a good story, however, that many fans may choose to believe it anyway.
Bellatrix At The Astronomy Tower
A great many Death Eaters witnessed Snape killing Dumbledore, but Bellatrix was not one of them. In the movie, however, the writers decided to include her. She pushed Malfoy to complete his task and cackled in delight when Dumbledore finally toppled over the tower's edge. She then sings and dances in delight as the group of Death Eaters escape.
Bellatrix was one of Voldemort's most trusted followers, so it did make sense that she would have been there. She was also invested in Draco's mission to murder Dumbledore, so it's easy to see why some fans might have forgotten this change.
Snapping The Elder Wand
In Deathly Hallows, Harry resents the power of the Elder wand. In both the book and the movie, he chooses to ensure that it could never pass on to anyone else. In the movie, he breaks it in half, and in the book he puts it back in Dumbledore's grave, intending to die before anyone else could claim it, eliminating its power.
In truth, the latter solution makes very little sense when everyone in the Great Hall saw him use the Elder wand, and any instance of getting disarmed would transfer the wands elegance. While breaking it in half didn't seem to prove a much better solution, it at least provides a more final elimination of the wand, which is why it might have solidified itself in people's minds as the canonical end.