Other mistake: After Ginny sees Hedwig and Harry's trunk at the Burrow, everyone gathers at the staircase and talks to Ginny about Harry's whereabouts, and when Hermione exclaims "Really?" we hear Harry answer "Really!" but the subtitles read that Ron answers Hermione. (00:13:00)
Other mistake: While Harry, Ron, and Hermione are walking back from the Three Broomsticks, Ron whispers to Harry about Hermione's earlier comment about snogging, and when we hear Ron say, "As if!" the subtitles read that Harry said it. (00:51:45)
Other mistake: In quite a few shots during the film, it is much too obvious that there are no lenses in Harry's glasses, whereas in previous and following shots the glasses have lenses. A couple of instances are: when Harry drinks the Felix Felicis and when Harry kneels beside Dumbledore, at the foot of the Astronomy Tower.
Other mistake: In the film Harry Potter and the Half Blood Prince, Professor Slughorn (during one of the 'Slugclub' meetings) asks Hermione what her parents do for a living, she replies, 'They're Dentists.' Slughorn and the other wizards stare blankly, not knowing what Dentists are. However, in the film 'Harry Potter and the Chamber of Secrets' in the burrow, we see the magical clock that shows the family's whereabouts, and we see that there is a section marked 'Dentist'.
Other mistake: When Harry and his friends visit the Weasley's house the second time - when the Death Eaters attack - the constellation Scorpio is visible in the sky. Scorpio is a summer constellation. It shouldn't be visible in the winter. Furthermore, it probably can't be seen as far north as England (although I don't live there, so I'm not sure).
Other mistake: When Professor McGonagall is talking to the trio about the necklace incident and Professor Snape is walking by in the background, he stops just before she calls him over.
Chosen answer: Snape wants to know how Harry knew Sectum Sempra. As shown by him using Occumelancy to see where his copy of Advanced Potion making is. But at the same time, if Snape punished Harry for a spell in a book, Harry could show it to Dumbledore, who would recognise Snape's handwriting. Additionally, Harry was certainly punished in the book - he got detention with Snape every Saturday for the rest of the term. The movie most likely had to cut this for time issues, and also perhaps because the scene is more dramatic if Snape only silently glares at him.
Ssiscool ★