Continuity mistake: In the Great Hall before the Quidditch match, in the shots facing Hermione she is sitting opposite the empty place setting beside Ron, but in the shots facing Ron she is directly opposite him. Take note of the goblets, plate of eggs, etc.
Continuity mistake: When Dumbledore approaches Slughorn, who is disguised as a chair, both the sheer and opaque curtains' appearance changes back and forth - also note the holes or lack thereof, between Slughorn's close-ups and wideshots. (00:07:00)
Continuity mistake: Aboard the Hogwarts Express, when Luna finds Harry lying petrified on the floor and then speaks with him, the way she holds her wand and the Quibbler newspapers differs between shots. (00:27:40)
Continuity mistake: In the scene before the Quidditch match, while Ron, Hermione and Ron are talking and Ron believes that Harry put something in his drink - in some angles Ginny is sitting directly next to Harry with their arms only an inch or two apart, in other angles Ginny's arm is nowhere to be seen, when they are sitting close enough that she should be visible.
Continuity mistake: When Harry is in the diner and places the newspaper down on the table as he speaks with the waitress, it lies over the table edge (front and side edges), then doesn't, then does so again... (00:04:05)
Continuity mistake: At the Burrow, when Harry is speaking with Lupin, Tonks, and Arthur, and then when Ginny joins him on the sofa, the orange blanket lying on the sofa's arm to Harry's right moves forward, back, forward, etc. (01:15:00)
Continuity mistake: When Dumbledore and Harry first arrive in front of Slughorn's temporary residence in Budleigh, Babberton, Harry's height barely even reaches Dumbledore's shoulders (likely the actor's stand-in), but in the next shot facing them Harry's height compared to Dumbledore is back to normal, level with the Headmaster's eyes. (00:05:35)
Continuity mistake: At the Three Broomsticks, when Slughorn and Harry speak, Slughorn holds a silver mug with a high foam head, but in the shot facing Harry, before Slughorn actually spills the drink the mug's foam head is gone (visible at the bottom of the screen), only to reappear when he spills it on Hermione ,whereupon the same previous angle is shown again without the foam.
Continuity mistake: During the first potion class scene during the movie, Hermione and Lavender are seen standing next to each other while being taught potions by Professor Slughorn. However, within the first minute or so of the lesson, Hermione and Lavender are standing far apart, then in the next shot, they are standing close together, with what looks like barely any space to move. They remain like this for the rest of the scene, but you can see the gap between the two within the first few minutes of the class.
Continuity mistake: In the Great Hall, from the time Hermione hits Ron with the book to when Harry (with bloody nose) joins them, the spoon in Hermione's bowl repeatedly faces the opposite way depending on the angle of the shot. (00:29:35)
Continuity mistake: The very first shot of Malfoy pulling the tapestry off the Vanishing Cabinet (after tossing a green apple in his hand) is used once again later (after Harry and Luna head off to Slughorn's party), when he is pulling the tapestry off again before placing a green apple in the Vanishing Cabinet. Note the same poof of dust, angle in which tapestry falls, and Draco's body position.
Continuity mistake: When Dumbledore and Harry enter the muggle house to find Slughorn the lights are all out; the only illumination comes from their wands. They enter the hallway, then both turn towards the living room with their wands in front of them. Despite this, the wall behind them is illuminated without either of them casting a shadow. The lighting in the hallway is obviously coming from a different source, not the wands. Can't be moonlight - it is a bright white light shining directly on the wall that was not visible when Harry and Dumbledore first entered the flat.
Continuity mistake: At the Three Broomsticks, when they notice Dean and Ginny snogging, Hermione's left arm is leaning across the table in front of her, then both her arms are down at her sides in the shot facing her, but next shot, her arm is on the table once again. (00:50:25)
Continuity mistake: When Dumbledore and Harry approach the Slughorn armchair, on the left there is a white lampshade lying on the shelf hanging over the edge. When Slughorn stands up and walks to the door to lock it, the lampshade is farther back on the shelf.
Continuity mistake: When Ginny snatches 'Advanced Potion Making' from Harry in the common room, and says "Who's the Half-Blood Prince? . That's what it says right here: 'This book is the property of the Half-Blood Prince'", she opens it at a random page in the middle of the book. When Harry sees the words 'This book is the property of the Half-Blood Prince', they are written on the first page. Also, the continuity is wrong as she is shown to switch pages between very quick shots.
Continuity mistake: At the end when Harry moves the hair from Dumbledore's face in the closeup, several strands of hair are left. When they pan back to Dumbledore, different strands of hair are across his face.
Continuity mistake: When Ginny rips the Half-Blood Prince's potion book out of Harry's hand, before Harry gets up from leaning against the couch on the floor, Ginny is nowhere to be found in the chair on the very far left of the screen. As Harry walks around the couch while talking to Hermione, she is suddenly sitting in the same chair.
Continuity mistake: At the Weasley's house when Hermione, Ron and Harry are talking about Dumbledore, Hermione's legs and the little table change position between shots. (00:14:30)
Continuity mistake: In the scene where Dumbledore and Harry meet Slughorn in the desolated house, some piano hammers lie on the ground. These hammers are from an upright piano but the "restored" instrument later in the scene is a grand piano. (00:06:25)
Continuity mistake: Tied round Dumbledore's beard is a thin silver strand, with dangling silver balls, that changes position and the height of the silver balls differ in consecutive shots, depending on the camera angle throughout the film. For example, when Dumbledore and Harry first pay a visit to Slughorn.
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 ★