Continuity mistake: During the second task, just as Krum and Hermione pop up from underwater and Krum's sharkhead disappears, in this specific shot Fleur (NOT another Beauxbaton student) stands on the platform in the background and her hair is fairly dry, blowing a bit in the wind. However, in a following close-up, Fleur's hair is soaked and stringy.
Continuity mistake: In the Great Hall, when Harry dribbles on himself, the plate to Hermione's right (as she holds The Daily Prophet up) has food and utensils lying on it. However, in the following shots, there is only an empty plate with utensils set on both sides, beside Hermione. (01:05:00)
Continuity mistake: In the Great Hall, after Hermione quips, "Well spotted!" Ron imitates his brother (when he asked Angelina to the Yule Ball) by lifting up his arms to mimic dancing, and Snape promptly smacks the back of Ron's head. In the next shot, Ron holds a pencil in his formerly empty right hand. (01:11:15)
Continuity mistake: During breakfast in the Great Hall, after Harry dribbles on himself, he and Ron are seated right beside one another and likewise their plates. When Nigel brings the box, all of a sudden there is a huge gap between them and a convenient open space on the table to place the box. (01:05:05)
Continuity mistake: After the 3rd task, as Moody is taking Harry back into the school, the camera cuts back to the students. Neville stands to the left (viewer's right) of Viktor Krum and the Bulgarians. In the next shot Neville stands to the left (viewer's right) of a girl, who is seen in the next shot standing beside Ron, Hermione and other students (not the Bulgarians); Neville is out of place during these shots.
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)