Continuity mistake: In the dormitory, when lovestruck Ron is going gaga over Romilda, he is wearing a brown sleeveless tee shirt and its tan trim has many holes. When Harry brings Ron to Slughorn, Ron is still wearing that tee shirt, but now some of the holes are gone and the rest of the holes have changed position. (01:26:50)
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: When Harry is kneeling beside Dumbledore's body, at the foot of the Astronomy Tower, Ginny joins Harry and holds him at his left side (viewer's right). In the last close-up of Professor Dumbledore's face, Harry is visible in the background with a space to his left and Ginny is now gone. That same angle was shown before Ginny joined Harry.
Continuity mistake: When Hagrid and Slughorn are drinking after Aragog's funeral, Slughorn's cup is considerably larger than Hagrid's (an effort to show size difference between Slughorn and Hagrid), and note when Slughorn holds his cup how large it is compared to his hand. But after Hagrid passes out Slughorn's cup is now the same size as Hagrid's cup - Slughorn's hand is resting on the table right beside the cup, which is now significantly smaller compared to Slughorn's hand. (01:50:05)
Continuity mistake: When Harry and Dumbledore return from the Horcrux cave, Dumbledore sits down with his arm around Harry's neck. When Dumbledore asks Harry to go fetch Snape, his arm suddenly disappears, and there was no time for him to have moved it. (02:10:00)
Continuity mistake: When Harry, Ron and Hermione are at the Three Broomsticks talking with Slughorn, Slughorn spills his butterbeer on Hermione and the table. Still in the conversation, the table is shown, but it's dry. Then a little later, there is foam on the table but only near Harry's glass. The rest of the table is still dry.
Continuity mistake: In the dormitory, when Harry walks in to see lovestruck Ron at the window, Romilda's card and empty chocolate wrappers are on Harry's bed. When Harry places the Marauders map and Potions book on the dresser, the card and wrappers have changed position. (01:26:55)
Continuity mistake: At the Three Broomsticks, when Slughorn spills the drink the foam by his fingers disappears, reappears, disappears, and so on.
Continuity mistake: When Slughorn and Harry are talking at The Three Broomsticks, Slughorn's grip on the silver mug repeatedly changes. His fingers are either at the top of the mug or at its center, depending on the camera angle.
Continuity mistake: In the living room at the Weasleys' house, while Harry, Arthur, Nymphadora and Lupin are talking, in the shot facing Lupin, Harry's shoelace is untied. After a brief cut to a shot facing Nymphadora and Arthur (where we can see Harry's hands), it cuts back to the original angle and now the shoelace is tied. (This has nothing to do with Ginny tying Harry's shoelace much later on.)
Continuity mistake: In the scene with Weasley's Wizarding Wheezes, as the camera pans up the front of the store, one sees the store name (in gold lettering). However, when Harry, Ron, and Hermione leave the store to follow Draco Malfoy the store name is no longer present.
Continuity mistake: Near the start, when Slughorn shows Harry the framed photographs, the frames change position between the wideshot and close-ups.
Continuity mistake: During dessert at Slughorn's supper party, the napkins that hang from some (Belby, Neville, etc) of their collars change positions in consecutive shots. (00:56:50)
Continuity mistake: As Harry and Luna come through Hogwart's gates, an Auror standing outside begins to walk backwards in through the gate. He makes it half-way through, but in the next shot, he is still at the end of the gate.
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.)
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.
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 ★