Revealing mistake: Just after Harry attacks Snape at the Shrieking Shack, he insists on knowing about Peter Pettigrew. In the next shot, as Lupin says, "He was at school with us. We thought he was our friend," there are wedges under the wheels of the piano, however, in the following shots they're gone. (01:34:10)
Revealing mistake: Right at the start when Harry is on his bed and making a light under his sheet, you can see that he is actually on something like a light panel in order for this effect to happen. For one thing, when he moves around on it, it doesn't bend like a mattress does. Also, about halfway down the bed you can just see where the sheet ends and this panel begins. (00:00:50)
Revealing mistake: After Harry's first ride on Buckbeak during Hagrid's class, before Buckbeak cuts Malfoy's arm with his talon, when Malfoy holds his arm over his face, the pre-cut and ripped sleeve is visible in this close-up. (00:37:55)
Revealing mistake: When Ron and Hermione are arguing with Malfoy near the Shrieking Shack, Malfoy visibly flinches before he is hit by the first snowball. He's not looking in the right direction to see it coming. (01:02:15)
Revealing mistake: When Harry first opens the Monster Book of Monsters, when looking closely, you can see the hollow part of the book that spits out the paper. Easiest to see when Harry is looking under the bed. (00:15:40)
Revealing mistake: When Harry and Hermione have gone back in time and they are running across the courtyard beneath the clock tower, you can tell this shot is using CGI because they seem to be running faster and further than the number of steps they are taking. (01:44:05)
Revealing mistake: In the scene in which Harry, under the guise of the Invisibility Cloak, is dragged by the Weasley twins into the castle by his arms, it's obvious that the twins are miming a person in between them. There is one point when they round the corner right before throwing him onto the stairs, the twins are pretty much side by side, there is absolutely no room between them for a person at that point. They then widen the space again right after they round the corner. (00:56:55)
Revealing mistake: When Hermione fakes reading the crystal ball, Trelawney insults her and Hermione knocks the crystal ball off the table. As the crystal ball leaves its stand it turns over and the tube used to create the smoke effect inside the ball is visible. Then as the ball rolls toward the stairs, there are no tubes visible. (01:19:55)
Revealing mistake: In the first overhead shot as the Dementors swoop down towards Sirius and Harry, Sirius lies in a particular position. During the Dementor attack his clothes move, then after Harry's Patronus repels the Dementors, Sirius breathes again and he now lies flat. After Harry collapses, in the last overhead shot, Sirius and his clothes lie in precisely the same position as the first overhead shot. His position differs later, in the repeat timeline as well. (01:42:50 - 01:45:20)
Revealing mistake: The first time the bus conductor shows Harry the newspaper with the picture of Sirius Black on the front, one of his fingers disappears "underneath" the moving picture for a split-second.
Revealing mistake: During the DADA class, as the Boggart balloon flies by, Lupin's eye contact with the balloon, as well as others', are not quite in sync as they follow the balloon with their eyes. (00:44:15)
Revealing mistake: When Neville opens his 'Monster Book of Monsters' he falls to the ground. Then he is lying on the ground and his book is attacking him again. We can see the pieces of pre-cut papers, that should be coming out of the book's mouth, actually come up from the ground, when Neville is holding the book above him. Obviously it's not real. (00:31:15)
Revealing mistake: When the students are running towards the portrait of the hippos where the Fat Lady is hiding, watch the railing of the stairs. It shakes back and forth, showing it isn't really made of stone.
Revealing mistake: When they look at the newspaper about Sirius being sighted you can see the picture of Sirius on the newspaper doesn't move at all. (00:37:55)
Revealing mistake: During the DADA lesson with the Boggart, the body double for David Thewlis (Lupin) is quite evident in some close range, head on and side shots. As a matter of fact, Emma Watson's and other actors' body doubles are very noticeable in many shots not involving a stunt. (00:44:15)
Revealing mistake: When Malfoy and his friends are messed with by an invisible Harry, Malfoy is dragged along by Harry. In the SECOND shot, when Malfoy is dragged along (which is a closeup shot of Malfoy's legs up in the air) pay attention to the bottom right corner of the screen, and you can see a bare hand quickly move into view, quite close to the screen. This can't be Malfoy's hand because he has gloves on. Slo-mo is not necessary to see this. (01:00:15)
Revealing mistake: While being hurled around by the Whomping Willow, Hermione grabs hold of Harry's shirt in the close-up. In the next shot as Harry's being lifted up, the bottom of the left side of his blue tee shirt is pinned to his pants, also visible in another shot. (01:29:30)
Revealing mistake: While he sits by the Whomping Willow, Ron shouts, "Harry, Hermione, run! It's the Grim!" When Sirius the dog runs toward them, in the shot facing Harry and Hermione, their eyes are not quite in sync with the dog before it leaps. (01:28:00)
Revealing mistake: During the Knight Bus scene, Harry smacks into the glass twice. Both times this happens, it's the same shot used twice.
Revealing mistake: After Ron has been dragged down the hole by the Grim, you see both Harry and Hermione get hit by The Whomping Willow and they fly backwards and land on the ground. Notice when Harry places his left arm in the ground, the ground is unnaturally soft. (01:25:00)
Chosen answer: Fred and George used the map to aid them in their mischievous activities at Hogwarts, and probably had little interest in what their little brother was doing or who he might be with at any given time.
raywest ★