Accidental Aesop: Hermione's subplot could be interpreted as, "Doing well in school is great, but your mental health comes first. No matter how smart you are, you can still bite off more than you can chew, so if school is putting a strain on your well-being, it's ok to dial back a bit."
Sir Cadogan, the portrait that takes over guarding Gryffindor Tower for The Fat Lady after she's attacked by Sirius Black, and whose work style involves setting "ridiculously complicated" passwords, and changing them at least twice a day. Is he stark raving mad, or Properly Paranoid taken Up to Eleven?
Snape's behaviour in this novel, where he's at his most emotional and unhinged has drawn a lot of views. Notably fans invoke information from later series to explain why he was so personally angry at Lupin. Others however note that his taunting at Harry by using Lily's own insults about James to the latter, and then having the latter remind him, much as Lily did in The Prince's Tale that James did save his life, and repeating the same conversation, while in the finale shouting at Harry that James deserved to die because he was too arrogant to trust Sirius is more or less him vicariously trying to argue that Lily was a fool for choosing James and wondering What Does She See in Him?.
At one point in Harry Potter and the Philosopher's Stone, Scabbers bit Goyle on the knuckle (which was even referenced by Fred and George Weasley in Azkaban). Come Azkaban, and the revelation that Scabbers was actually Peter Pettigrew, who was the true person responsible for leaking Lily and James Potter's location to Lord Voldemort and thus their deaths, not to mention the revelation in Goblet of Fire that Goyle's father was most likely one of Pettigrew's comrades among the Death Eaters it becomes somewhat disturbing.
Ron carrying him around in his pocket and Scabbers even falling asleep on Harry's bed at one point in Philosopher's Stone. Counts in-universe too as when Pettigrew tries to plead with Ron for mercy, Ron (with appropriate anger and disgust) points out he let the rat sleep on his bed.
Growing the Beard: Many fans believe that this is the turning point for the series as a whole, citing the encounter at the Shrieking Shack, the darker emotions and actual feelings of loss Harry feels about his family (that had not been really explored in the previous books) and of course the Bittersweet Ending. Sirius is proven innocent to Harry and Dumbledore but remains on the run alongside Buckbeak, and Lupin has to quit, while Harry worries about Trelawney's prophecy and the consequences of sparing Wormtail potentially bringing Voldemort back. Compared to the out-and-out positive victories in the first two books, it was bleaker and prepared for the gradual darkening of the remaining books.
The part where Snape gets insulted by the Marauder's Map after Pottermore revealed that it was designed to do that to Snape and only Snape, not just anyone who tried to read it without the password.
In the novel, Hagrid expresses worry that Buckbeak might have encountered werewolf!Lupin in his escape from Hogwarts. As the movie illustrates, Hagrid need not have worried for Beaky's welfare in such a fight.
While discussing Sirius Black Hagrid says that if he had found Sirius before Pettigrew did he would have 'ripped him limb from limb'. Fudge dismisses the idea saying only a squad of top Ministry wizards would have stood a chance against him. Two books later we see a fight between Hagrid and a squad of Aurors. Hagrid wins. Easily.
Idiot Plot: The denouement at the end depends on the following characters (Snape, Remus) suddenly forgetting the following details: Remus is a werewolf, tonight is a full moon, and Remus has to take the Wolfsbane potion. Remus forgets about it entirely, despite the fact that he tok it for the last year note he specifically says that he isn't too good a potion maker and the Wolfsbane Potion is very complicated, also he was very poor so he probably didn't have the ingredients, which can only be explained in a character sense about how seeing Sirius and Peter Pettigrew on the Marauder's Map makes him think differently. Snape however comes to Remus room with the Potion (which he mentions during their confrontation) and despite having had a traumatic experience in his youth at the Shrieking Shack, still goes down there to meet Remus without having any real backup plan against a werewolf set to transform. Without this contrivance, Peter Pettigrew would not really be able to escape and Sirius still be on the run. Much also depends on the fact that Lupin tied up Pettigrew instead of hitting him with a Petrificus Totalus and moving him along by magic like Sirius did Snape. Of course a conscious Pettigrew would jump on the diversion made by Lupin transforming first chance he gets, grabbing his wand and escaping.
It Was His Sled: Sirius ain't evil. Lupin is a werewolf. Ron's pet rat Scabbers is really Peter Pettigrew, a human.
Hermione's white face.note The only line in the entire series indicating Hermione's skin color, though several other times she is described as pale, and she is depicted as white on some front covers. Used as a justification for fans that were surprised at the casting of a black actress in Harry Potter and the Cursed Child, notably when J.K. Rowling claimed she'd never mentioned Hermione's race, even though theater casting has been color-blind like this for a while now; for example, there are adaptations that make Romeo and Juliet black too, even though they are supposed to be medieval Italians.
Snape's over-the-top final rant to Fudge after Sirius escapes, "This has something to do with Potter!" is often invoked as one of his catchphrases.
Never Live It Down: Among a lot of fans, Snape never quite recovers from this book, even after re-reading the series with all his motivations revealed. He's wildly unprofessional, sadistically and openly cruel even to students like Neville to a level that would have gotten any teacher fired from a real-life school, was willing to send Sirius to the Dementor's Kiss even after he was open to turning himself in to Dumbledore, and then outed Remus Lupin as a werewolf publicly. Even the revelation from the later books about Sirius bullying Snape as a young man and Snape being loyal all along doesn't vindicate his conduct, since his incompetence, i.e. neglecting to bring or mention the Wolfsbane Potion to Remus after entering the Shack, played a major part in letting Peter escape, a detail that slipped him since he wanted to get back at Sirius and Lupin. His behaviour is so bad that the movies had to tone him down to not make him too unsympathetic.
One-Scene Wonder: Aunt Marge only appears in one chapter (which has her name on it), but still manages to be among the more despicable characters in the series, perhaps even more so than her brother Vernon.
Shipping Goggles: The series has enough of this to fill a page, but this entry probably has the best example, what has been dubbed "The Symbolic Flight". This theory states that because Harry and Hermione flew together on Buckbeak together and without Ron, that they are destined to be together - ignoring the fact that Ron was only not present because he broke his leg earlier.
YMMV tropes present in the film:
Accidental Innuendo: The scene from the book listed above is even more dodgy in the film, when Harry's Uncle Vernon keeps bursting into the room to find out what's going on. Vernon is played by Richard Griffiths, AKA Uncle Monty.
Base-Breaking Character: The Shrunken Head on the Knight Bus. Some find him bizarrely out of place and unnecessary, others find him a nice bit of comic relief. It's worth noting that it was approved by JK Rowling - who prevented Alfonso Cuaron from inserting a choir of tiny people playing the piano.
Better Than Canon: The Adaptation Expansion for the Time Turner sequence is widely agreed to be a big improvement over how it was handled in the books. A few readers missed the Foreshadowing, and thus found the twist to come really out of nowhere. The film expands on this - having Hermione suddenly turning up out of nowhere in classes at points throughout the film. Also the twist that Harry himself conjures the big Patronus is Foreshadowed by having more events influenced by the time travellers - throwing the pumpkin seeds to warn that Fudge is on the way for example.
After everyone arrives at Hogwarts to start the new year, the Fat Lady won't let them into Gryffindor Tower because she's too busy trying to shatter a glass with her voice.
A lot of screentime is devoted to a blue bird flying through Hogwarts and twittering around; far longer than normal for a scene transition. The deleted scenes show that the sequence ran even longer - with the bird buzzing around Hagrid's head before getting shooed away.
The choir singing an arrangement of the witches' chant from Macbeth... along with croaking toads?
It should be noted that the first two here do serve some purpose in introducing the Fat Lady and the Whomping Willow tree (or just refreshing viewers' memories for those familiar with the series) so that they don't come out of nowhere when they become relevant to the plot. But the frog choir? Yeah that's just straight up Hogwarts BLAM.
Broken Base: Half the fanbase considers this one of the best (if not the best) Harry Potter film in the franchise, or at least when the Harry Potter films started getting good. The other half considers this movie to be one of the worst (if not the worst) Harry Potter film in the franchise. The film was mostly better received by critics than the first two, although it was the first HP film not to get four stars from Roger Ebert. Also, this is J.K. Rowling's favorite film. General consensus seems to be that the cinematography and visual effects are excellent, but the acting and script bring it down in places.
Continuity Lock-Out: This was the point when the movies started to get hard to understand for anyone who wasn't already familiar with the respective books. Most notably, this movie never makes it clear who Padfoot, Wormtail, Moony, and Prongs are, despite later movies operating on the assumption that the viewers all know.
Critical Dissonance: The film was adored by critics when it came out, opening to the most critical success any Potter film had seen at that point. On the other side, it left the Potter fanbase fairly polarized.
Darker and Edgier: Much more so than the previous two films, signalling the shift in tone for the remainder of the series.
Growing the Beard: According to most professional critics, along with about half of the fans. At the very least, people agreed that the younger actors and actresses were finally fitting snugly into their roles (aside from the Narm mentioned below). What's more is that this was the first film to make deviations from the book in the name of Pragmatic Adaptation, paving the way for the future films to do so (without falling into In-Name-Only).
Harsher in Hindsight: Sirius's infamous line "I DID MY WAITING..TWELVE YEARS OF IT...IN AZKABAN" became this in 2018 with the release of the Allegedly Free GameHarry Potter: Hogwarts Mystery, which features an energy meter that, if empty, forces players to wait unless they buy Microtransactions.
Internet Backdraft: Best movie in the series? Worst movie in the series? Middlingest movie in the series? Whatever your opinion, a huge chunk of the fandom will loathe you for it.
Narm Charm: The examples in the above page, as well as "You foul, loathsome, evil little cockroach!" They're a near 50/50 mix of awesome and narm.
Never Live It Down: A lot of the fandom's complaints with Hermione stem from this film. Namely her getting Ron's line standing up to Sirius, Sirius telling her she's the smartest witch of her age instead of telling Harry he's truly his father's son, and her action scene on the Whomping Willow (despite the fact that she gets into a worse scrape than Harry in that one). If the fans ever complained about Steve Cloves turning Hermione into the Creator's Pet, expect this film to be mentioned as the root of it.
Special Effects Failure: At the very end of the scene where Lupin berates Harry for not turning in the Marauder's Map, there's an obvious (especially so when looking at the lighting) jump to a different take in the middle of a shot. It occurs just after Harry tells Lupin that he thinks the map might not be working since he saw Peter Pettigrew on it.
Alfonso's complete redesigning of Hogwarts geography, coupled with the overall more comedic tone the film has compared to the previous two, not to mention the Shrunken Head added to the Knight Bus.
The excision of the explanation of the identities of Moony, Wormtail, Padfoot, and Prongs, which confused many moviegoers who hadn't read the books. Especially since it would have been relatively simple to slip into Lupin's final scene.