Continuity mistake: There are two scenes during which Harry is shown having visions of the Dept of Mysteries while he is asleep. The first scene occurs after Harry's confrontation with Seamus in the Gryffindor common room. There are shots of Harry lying in bed wearing a short sleeve, crew neck tee shirt, but when he is startled awake by the vision of Voldemort, he sits up wearing a buttoned henley tee. The next day is the first day of Defence Against the Dark Arts class with Umbridge. That last shot of Harry in the henley tee shirt was cut from a later scene. Later in the film, when Harry has the vision of Arthur Weasley's attack, he is wearing the henley tee shirt.
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.)
Continuity mistake: In her office, when Umbridge adds sugar to her tea, she places the spoon on the saucer. When Harry walks in and she says, "You're going to be doing some lines today," there is a close-up of the tea cup and the spoon is gone. After a brief close-up of Harry, the spoon reappears.
Continuity mistake: In the Hall of Prophecy, when Neville spots Harry's glass orb and says, "It has your name on it," he is looking up at the fourth shelf, above his head. In the next shot of Harry he is looking up as well, but in his next close-up, the orb is now on the third shelf, and Harry (who is at least a foot shorter than Neville) is actually face-to-face with it, before he ever touches it.
Continuity mistake: At the playground, when Dudley and Harry have a confrontation, Dudley wears black shorts with large white designs of words and numbers. When Dudley and Harry enter the tunnel and then later, when yumpy Dudley is in his house, Dudley's shorts have completely different designs (though it is the same type of shorts).
Continuity mistake: After Harry's vision of Arthur Weasley's attack, he has the first Occlumency lesson with Snape. The windows behind Harry have a diamond shape pattern, with horizontal and vertical divisions on the glass. In the last shot, as Snape orders Harry to focus, when the camera dramatically pans out through these windows there is only a diamond shape pattern on the glass.
Continuity mistake: In the Great Hall, when Dumbledore says, "Good evening children, I have two changes in staffing this year," we see Fred and George Weasley sitting in the fifth and sixth seats, and the other students near the end of the Gryffindor table. Next shot facing Dumbledore, we see the last seven students at that side of the table and not only are the Weasleys gone, but other students differ as well. This is also seen in additional shots.
Continuity mistake: In the Room of Requirement, before the Christmas holiday, Harry compliments the DA on a job well done, and they clap. There are two consecutive shots of the group from different angles, and between shots Cho's wand turns upside down and another girl's wand changes hands, etc.
Continuity mistake: In the Gryffindor Common Room, when Hermione and Ron sit down beside Harry in the first shot, Hermione's left shirt ribbon hangs over her shoulder under her hair. When it cuts to the front shot, as she sees Harry's hand, that ribbon hangs properly down the front of her shirt.