Continuity mistake: After Harry arrives at the Burrow, when he, Hermione and Ron are sitting and chatting, Hermione changes position between shots - notably her knee is up, down, up, etc. (00:13:45)
Continuity mistake: In the Room of Requirement, when Draco first uncovers the Vanishing Cabinet, the items on the floor behind him change position between the overhead shot and next close-up.
Continuity mistake: After drinking the poison in Slughorn's office, Ron falls and in the overhead shots the top of his head is near the rug's edge, but in side shots his head is well below the large diamond pattern, more than a foot away from the edge.
Continuity mistake: When Harry finds Hermione crying at the bottom of the stairs, with the conjured canaries flying overhead, there are numerous leaves/debris on the lowest steps and near her feet, but in the shots looking down from the top of the stairs the area is clear. (This has nothing to do with the few leaves that blow about.)
Visible crew/equipment: When Harry is lying outside the hut at the end of the film, Snape kicks his wand away. A block of neon tape can be seen briefly beneath the wand when Snape kicks it.
Audio problem: When Ron and Harry are walking down the corridor towards Potions class, Ron says, "...Quidditch trials coming up, I need to practice," but his mouth is not moving and/or not in sync with that dialogue.
Continuity mistake: After Hagrid passes out in his hut, when Harry tells Slughorn of Lily's sacrifice the vegetables behind Harry change position on the shelves. (01:50:40)
Continuity mistake: In the Great Hall, when Ron sits down with Harry and Hermione before Quidditch, the newspaper Hermione is reading goes from her hand to under her arms, then back again, etc, between shots.
Revealing mistake: Harry and Dumbledore apparate onto Professor Slughorn's street, and Harry says "I just apparated, didn't I?" As Dumbledore is answering, ("Indeed. Quite successfully too, I might add.") the shot pans out. Look at Dumbledore's "burnt" hand. The green glove they used to make his hand look blackened is visible on his fingers. It is also visible when he turns and starts walking away from the camera. It is not due to the lighting, as it is not visible in the next shot as they both walk away. (00:05:15)
Revealing mistake: In the office building at the beginning of the film, the scene is located inside a meeting room of the very famous Swiss Re tower in London. But at the same time, the camera shows the Swiss Re tower through the window. (00:01:20)
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)
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).
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.
Answer: The purpose of the Pensieve is to preserve the memory exactly as it was recorded. Telling Harry something years after an event occurred is not as effective or as accurate as seeing. It is more impactful if Harry can watch events exactly as they unfolded. Regarding Slughorn, he had refused to give his complete memory because he was ashamed of the unintentional part he played in Voldemort's plan to create the Horcruxes. He was duped by Riddle, but he still did not want anyone to know how foolish he was.
raywest ★