Continuity mistake: Outside Hagrid's, when Fudge says "We must search the grounds," the way Dumbledore holds his staff changes between shots. (01:50:15)
Continuity mistake: When Hermione says, "Awful things happen to Wizards who meddle with time, we can't be seen," in the shot facing her she places her right elbow up on the pumpkin. In the next shot facing Harry her arm is down on her lap, yet in the next shot facing Hermione her arm is up again. (01:50:55)
Continuity mistake: When Hermione and Harry hide behind the pumpkins, Hermione says, "Come on! We're coming out the back door. Go!" As HRH run out of Hagrid's, Hermione and Harry run to hide behind the trees. In the shot facing the two, with the pumpkins and Hagrid's behind them, the blanket on the ground beside the pumpkins is visible, though it is gone thereafter. (01:52:00)
Continuity mistake: After Ron, Hermione and Harry run out of Hagrid's they hide behind the pumpkins, while the other Hermione and Harry look on as they hide behind trees. When Hermione comments about her hair, the tree branch she moves away is completely different between the shots facing her and the shots facing Hagrid's. (01:52:10)
Continuity mistake: When Hermione and Harry go back, when they are at Hagrid's things lying in his front yard, such as the cauldron, wood, etc., change position in the following shots. (01:52:50)
Continuity mistake: When Hermione tries to coax Buckbeak to follow Harry she holds up the ferret in her hand. Between the shots facing Hermione and then Buckbeak, the ferret and its paws change the direction that it faces while she grips the hind of the animal. This is only in regards to when she grasps the ferret and does not let go. (01:54:00)
Continuity mistake: While Hermione tries to coax Beaky to follow her and Harry, in the wide shot she and Harry stop, she tosses the ferret and throws her hands up in the air. In the next two close-ups their positions differ. (01:54:05)
Continuity mistake: While Harry and Hermione try to coax Buckbeak to follow them, Hermione has the rope with the ferrets around her neck, which differs between shots. After Hermione tosses Beaky the second ferret, in the shot facing her she takes hold of one of the two hanging on the rope, on her left and dangles it, yet in the next wide shot she dangles both ferrets, in her hand. (01:54:15)
Continuity mistake: When Sirius flies off on Buckbeak, the ground on which he takes off from is a perfectly clean stone ground with nothing on it. Yet after Sirius has flown off, Harry and Hermione run across the same ground, only now it is covered in dead leaves, grass and mud, etc. (01:59:40)
Continuity mistake: The angles, length and position of Harry's scar on his forehead, change considerably throughout the film. (02:00:25 - 02:02:05)
Continuity mistake: At the end of the film when Harry gets on the Firebolt, everyone is running outside, and there is no crowd properly formed beside the water fountain, yet when Harry takes off there is a very large crowd watching him all of a sudden. (02:04:05)
Continuity mistake: After saying goodbye to Harry, Sirius climbs onto Buckbeak and Hermione hands the chain to Sirius. In the next shot Sirius holds the chain differently. (02:04:15)
Continuity mistake: When Harry is in Lupin's office, as the Professor says, "I resigned..." his cane leans against the corner of the trunk beside the doorway. Yet when Harry says, "But Dumbledore..." the cane hangs off the center of the trunk. As Harry says, "None of it made any difference," the cane is gone, but when Lupin says, "You helped uncover the truth," the cane hangs again. Then when Lupin says, "If I am proud of anything..." the cane leans against the corner, but wait, when Lupin leaves the office the cane hangs off the trunk once again. (02:06:10)
Continuity mistake: As Harry reaches Lupin's office, Lupin stands by the bookcase and the front end of the valise under the desk points towards the right. However, when Lupin says, "No, no, I resigned," the front end points the opposite way. Then when Harry walks to the doorway, it's the same way as the first shot. (02:06:10)
Continuity mistake: When Harry first reaches Lupin's office, as Lupin is packing, the Marauder's Map lies open on the desk. The map folds change position when Lupin says, "I saw you coming." (02:06:10)
Continuity mistake: As Lupin packs in his office, the gramophone and books on his desk are visible in many shots. When Harry first walks in, the set of three thin books, bound with a string at the top of one pile, as well as the gramophone and other books noticeably change their position in quite a few shots. (02:06:10)
Continuity mistake: As Harry approaches Lupin's desk, the open trunk is visible to his left, near the window. When Lupin tucks his belongings away in his trunk, items such as the books are positioned differently. (02:06:30)
Continuity mistake: While Harry is in Lupin's office, Lupin's coat, which lies on a trunk, changes its position multiple times, as does the trunk. (02:06:30)
Continuity mistake: In Lupin's office, the chair, vase on the floor, etc., between the window and fireplace significantly change positions. Noticeable when Lupin says, "This time tomorrow the owls will start arriving..." then continues, "Dumbledore has already risked enough on my behalf," to the close-up when he magically packs his belongings in the trunk and thereafter. (02:06:40)
Continuity mistake: When Harry first arrives at Lupin's office as he packs, the skulls with the horns, sitting atop the bookcase, are visible. In the following shots, the skulls not only drastically alter their positions, the one with the very tall horns is actually gone in some shots. (02:07:45)
Answer: This didn't happen in the book. It appears to be done purely for comic effect in the movie, showing Dumbledore's eccentric and quirky nature. He's seemingly oblivious to what he's doing and how it affects Ron.
raywest ★