Continuity mistake: When Ron and Hermione are arguing about Crookshanks in the Leaky Cauldron, Sirius does not move in the 'wanted' poster. That same poster is visible when Harry first enters with Tom, and Sirius doesn't move in that shot either. However, when the poster is the focus of the shot, Sirius does move in the magical poster.
Continuity mistake: When Malfoy shouts "Dementor. Dementor." all the kids turn around. Then we see Malfoy and some kids put their hoods on, and at the same time we can see Hermione turn back. When it cuts to Potter turning back we can briefly see Hermione turn back again. (00:31:45)
Other mistake: When Harry gets the map from Fred and George, he watches it and then one of the twins taps it and the contents fade away. But the text to the left, in shadow of the folded pages, doesn't fade. (00:58:15)
Revealing mistake: In the punch scene "replay", Harry is noticeably stockier, and I would suggest shorter, and he walks in a very unusual way - very different from Harry. Obviously a double.
Continuity mistake: When Potter points his wand at Uncle Vernon you can see his index finger on the wand. When it cuts his index finger is pulled back. [Harry has obviously jerked backward when Uncle Vernon approaches him -- Vernon is obviously closer in the second shot -- and it would be a natural reflex to withdraw his finger as well.] (00:06:25)
Continuity mistake: In the beginning when Aunt Marge says, "The bitch and the pup" and Harry yells at her, Mrs. Dursley turns to look at Harry. When the shot changes, Mrs. Dursley is now facing the table, away from Harry. (00:04:00)
Revealing mistake: When Mr Weasley is warning Harry about Black wanting to kill him there is a jug in the background supposedly magical because it is serving the drinks by itself. If you look closely you can see the wire or string or whatever held it up.
Continuity mistake: In the scene when Hermione punches Malfoy in the face, she holds her wand to his neck with her right hand. The shot changes to her turning away and then she punches him - both her hands appear in the shot and her wand is gone. In the next shot she mysteriously has her wand in her left hand with no time to have dropped it then picked it up again.
Continuity mistake: When Harry gets his turn to face the Boggart in Lupin's class, there is a wide shot showing the large clown rocking back and forth (viewer's left to right), with Lupin in the background bent over looking down at the floor. But, in the next close-up, Lupin is not as bent and is suddenly facing clown, contemplating whether or not he should do what he does next.
Continuity mistake: At the dinner table when Aunt Marge says "Vernon, do something" the cloth in front of her on the table is fine. When it cuts the cloth is crinkled. (00:04:30)
Revealing mistake: At the Shrieking Shack, when Harry shouts, "Expelliarmus!" there is a nice close-up of Snape and it is obviously not Alan Rickman flying backwards onto the bed. (01:34:00)
Deliberate mistake: The wings of the hippogriff are much shorter when folded up than when flying. According to the animators, the wings would have to be huge to carry the weight of the hippogriff when flying, but so big that they would drag on the ground when folded up. So they deliberately made the wings shorter. (00:33:30 - 00:35:55)
Continuity mistake: When one of the Weasley twins says, "Not flashing that clipping again." to Ron, we see Ron holding Scabbers. The way he holds Scabbers changes in the next shot. (00:16:20)
Visible crew/equipment: When the student's are looking at the wardrobe while listening to the teacher you can see a crew member on the glass window of the door of the wardrobe.
Continuity mistake: When Potter has entered the Knight Bus you see him passing a bed. While doing so he grabs the bed pole and lets go of it, but when it cuts you can see him still holding the pole. (00:08:55)
Audio problem: When Harry and Hermione have gone back in time and are watching the Dementors attack, Hermione says, "You're dying." When she continues, "Both of you", her mouth is moving before the words are heard. (00:56:05)
Continuity mistake: In the Leaky Cauldron, Harry stares out his room's window as a train passes. At the start of the next shot, as he walks toward the Monster Book of Monsters, the dresser to the left of the fireplace is visible. In the following shots the large water pitcher and wood box reach the edge of the dresser, whereas in the previous shot it is nowhere near. (00:14:55)
Audio problem: When Hermione says "I thought I just saw. Never mind" the second time, it is in a much lower tone than when she first said it.
Continuity mistake: When Aunt Marge is inflating and floating to the ceiling, she grabs the table cloth and continues going up and take the cloth with her. Then when one of her shirt buttons pops and hits Dudley you can see that the cloth is not moving but has slid across the table a bit. It cuts to a close up of Marge and the cloth does not move, it then moves to the wide shot of the whole table and the cloth continues to move. The other button pops, hitting Dudley again, and the cloth stops moving again. (00:04:55)
Continuity mistake: In the scene where Sirius lies at the lake and Harry kneels next to him, Sirius seems unconscious, then opens his eyes in terror as he sees the Dementors. However, in the very next shot, which shows the two from above, his eyes are suddenly closed and there's no sign of movement in his face. Too big a change, too fast. (01:38:45 - 01:42:50)
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 ★