Continuity mistake: In the Great Hall, when Ron (wearing his Quidditch uniform) asks Hermione about the Slug Club, Ron's right arm lies on the table in front of him in shots from behind, but in shots facing him his right arm is down beside him.
Continuity mistake: When Hermione opens the locket to read the note, the locket and chain's position changes in her hand between consecutive shots.
Continuity mistake: At the end of the movie, there is a long shot of Hermione, Ron and Harry atop the Astronomy tower. In this long shot Hermione's hands are in different places compared to the previous close up.
Continuity mistake: During the Slug Club's dessert, as Slughorn speaks to the students, their ice cream, syrup, thin wafers, spoons, etc, change depending on the camera angle.
Continuity mistake: In Fred and George's joke shop Ron asks how much something is, the twins reply 5 galleons, one of the twins (closest to Ron) is leaning against the banister then in the next shot of him he is in a different position.
Continuity mistake: In the Christmas scene at the Burrow, while Harry, Lupin, Tonks, and Mr Weasley are talking about Snape: notice the trays of candies/baked goods on the table. When they are done talking and Ginny comes to sit down and give Harry a bite, a spot on the table has suddenly cleared for her to sit down her tray when Ron comes to sit in the middle of Harry and Ginny.
Continuity mistake: When Death Eaters reach Diagon Alley, there's not many people there; but after they crash there with Ollivander, the alley gets pretty crowded immediately.
Continuity mistake: When Ron sits down with Harry and Hermione in the Great Hall before Quidditch, the bowl/spoon is either right beside Hermione's left hand or farther away, depending on camera angle.
Continuity mistake: In Potions class, just before the students go to their cauldrons to prepare their Draught of Living Death, Hermione stands in front of Harry, but in the long shot she is instantly beside him, then she's suddenly back in front of Harry again.
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 ★