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.
Revealing mistake: While Hermione and Ron watch, cloaked Harry starts up with Malfoy and his two friends as they all stand in the snow. Most noticeable in the wide shot, when Harry spins the Slytherin boy, although the boy's feet visibly disturb the snow, Harry's feet do not make a mark - no footprints whatsoever.
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.
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.
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.
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.
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.
Revealing mistake: When Malfoy and his friends are beaten up by an invisible Harry, Malfoy is dragged along by Harry. In the SECOND shot when Malfoy is dragged along (which is a close shot of Malfoys legs up in the air) look closely (perhaps slow-mo) in the bottom right hand side of the screen and you can see a bare hand move into view, quite close to the screen. This can't be Malfoys because he has gloves on.
Revealing mistake: In the Shrieking Shack, just after Scabbers becomes Pettigrew, when Sirius and Lupin pull him out of the hole, as the camera faces Pettigrew's back while they hold him, the blue tape on his 'missing' finger is visible for 2 frames. (It is necessary to go frame-by-frame.)
Revealing mistake: As Harry and Buckbeak fly over the loch, Buckbeak's foot trailing in the water leaves a wake, as it should. This wake should also disturb the ripples and waves that are already present on the surface of the water, but they do not. It is obvious that the wake effect was added in, but the foundation shot of the loch was not edited to help it make sense.
Revealing mistake: In the scene at night, when Harry is in the hallway with the map, the tip of his wand lights up. And later in that same scene, when Snape has the conversation with Lupin and Harry, Snape's wand does the same. However, if you look closely at the shadows on their faces, they're being lit from above, not from the wands' light below them.
Revealing mistake: In the Shrieking Shack, when Harry sends Snape flying into the bed, the canopy collapses in the stunt. In this shot the metal bars at the top of the canopy frame are visible, whereas in the following shots it is no longer there. The way the canopy frame actually hangs after stunt differs considerably and the draperies hanging on the bed differ in the shots prior, during and after the stunt.
Revealing mistake: At Hagrid's lesson, just after Malfoy wound Harry up, there is a close up on the class when they first see Buckbeak and you can see Emma Watson holding back a laugh. Surely if her friend was being picked on it wouldn't be funny, plus they're supposed to be scared of Buckbeak.