Revealing mistake: Throughout almost the whole movie Harry's glasses don't have lenses in them. Only in two or so scenes do they appear.
Revealing mistake: In the Forbidden Forest, HRH are introduced to Grawp. Just as the giant lifts Hermione off the ground, in the shot from behind her, Hermione's legs disappear below her calves; the digital lines are actually visible. Slow-mo is not necessary. (Only visible on fullscreen DVD.) (01:29:40)
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: It is not what is seen here that is the problem, but what we don't see on the Hogwarts Express; first on the way to Hogwarts when Harry, Ron, and Hermione are in their compartment talking about Draco, and second, during holiday break, when Harry and Ron are talking about Unbreakable Vows. As the camera faces the large compartment window, we see the scenery out the window, but as the train speeds along, the scenic reflection of the windows opposite it, in the corridor, should be seen but isn't. What is reflected is the light colored wall of the compartment beneath the blacked-out window. The camera filming the shots from this angle is positioned at that window, and to hide the camera/crews' outline, the window was covered.
Revealing mistake: Aboard the Hogwarts Express, when Harry stands beside the open window and sends Hedwig off with the letter for Sirius, surely his hair and clothes would be wildly moving about, because of the strong wind from the fast moving train.
Revealing mistake: At the zoo, Harry looks to his right, at the 'bred in captivity' sign of the Python. In this close-up, the green plant that is below, to the side of the exhibit sign, is protruding out past where the glass supposedly is, yet this is BEFORE the glass disappears. As a matter of fact, this plant, nearest the exhibit sign, changes its position considerably throughout these shots. (00:06:50)
Revealing mistake: In the scene where Harry and co. are in the Room of Requirement, at several points during this scene, you can tell by the way the light shines that Harry's glasses are missing their lenses, especially in close up shots of Harry.