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: At the diner when Harry walks toward the windows, in the close-up of a window through which we see the distant platform (before Dumbledore appears), the style of the window's muntins (sash) around the glass panels is quite different in this close-up, than in the previous shots of the windows. (00:04:00)
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)
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)
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: 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: When Dumbledore brings Harry to meet Slughorn, the Daily Prophet is lying on the floor amongst debris, including the piano's hammer, which differs between shots. (00:06:50)
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)
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)
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: 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: Aboard the Hogwarts Express, when Draco petrifies Harry he falls immobilized. Just as Draco kicks him, Harry's head is positioned farther out in the aisle, than previous and following shots. (00:26:40)
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 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: In Potions class, when Hermione stands before the cauldron explaining what she is smelling in her closeup, we see Neville behind her with the broad space to his right (viewer's left), just as she says, "Spearmint toothpaste". In the instant cut to the wideshot, Harry is now shoulder to shoulder with Neville, too fast for him to suddenly be standing that close to Neville. (00:34:35)
Continuity mistake: In Potions class, when Slughorn removes the vial of Felix Felicis, he turns to face the class and holds the vial up for the class to see, but in the next shot facing him he holds only the very bottom point of the vial. (00:35:15)
Continuity mistake: At the Muggle orphanage, in the corridor when Mrs. Cole says, "Tom, you have a visitor," Dumbledore's scarf hangs askew, but next shot facing him the scarf hangs properly. (00:40:05)
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: At the Three Broomsticks, Slughorn is holding a mug with his left hand when he speaks to Harry, but when he says, "Good to see you, Wallenby," the mug is suddenly in Slughorn's right hand, with his left arm down by his side. (00:51:15)
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)
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 ★