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: On the train to school, when HRH are in their cabin, among the different angles that are presented during this scene are the shots in which the camera either faces the large cabin window or the aisle window, directly opposite it. It is not what is seen that is the problem, it is what we don't see. When the camera faces the large cabin window we should see the scenic reflection of the window opposite it, as the train speeds along (and vice versa the aisle window), but we don't. What we do see reflected is the light colored wall beneath the blacked out window. The camera filming the shots from this angle is positioned at that window and in order not to see the outline of the camera/crew, the window was covered.
Revealing mistake: At the first challenge, just before the Horntail makes its appearance, there is a tight close-up of the egg as Harry stands in the stone "doorway" in the background, on the left. Clearly it is not Daniel Radcliffe, but his stand-in (his face differs and he has a shorter, bowl-type haircut) who is standing there, for this one specific shot.
Revealing mistake: In the graveyard, when Voldemort puts his foot on Cedric's face, Cedric's eyes are staring straight up. As Voldemort pushes, Cedric's eyes do not follow his head's movement, but continue to stare at the point that would have been straight up from his head's original position.
Revealing mistake: During the Yule Ball there is a shot of Professor Moody sitting on a bench tapping time with the music. One can easily see a green object beneath the bench. It is apparent that a hole was cut in the cushion with his real leg hanging below the bench covered with a green leg and foot sleeve. The large green object is his foot with a red dot on his toe. In post production the actor's real leg and foot is to be removed digitally via green screen-like technology and replaced with a digitized artificial one. The red dots are to help coordinate the movements and position of the artificial leg with the actor's real one. This shot obviously got missed by the postproduction team. If you look closely you can also just make out his knee peeking below the fringe of the bench. (01:18:50)