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.
Visible crew/equipment: In the Room of Requirement, during the Patronus lesson Harry walks behind Ginny, who stands between two other girls, and she says, "Expecto Patronum!" As the Patronus exits her wand, just before the shot cuts away, in the reflection behind the girls we can catch a glimpse of the camera, cameraman and crew member wearing a short-sleeved tee, between and behind the reflections of Ginny and the blonde girl.
Continuity mistake: As Harry holds the Prophecy, when Lucius says, "Don't you want to know the secret to your scar? ...I can show you everything," both he and Bellatrix stand directly in front of Harry and neither are wearing masks. The problem is in the overhead shot, when Harry says, "I've waited 14 years," two mask-wearing Death Eaters are approaching Harry, who's holding the glowing Prophecy orb, but maskless Lucius and Bellatrix are supposed to be standing there already.
Continuity mistake: At 12 Grimmauld Place, when Harry and Sirius are in the room with the Black family tree, as Sirius says, "My parents with their pure-blood mania," the area of the wall with both Sirius's and Andromeda's names and burned off pictures is quite visible (as in other shots as well). Bellatrix's name is beside Andromeda's (her sister), but there is no picture over Bellatrix's name banner in this shot (only branches and leaves), nor in any other shot, except in the previous close-up, as Sirius says, "My deranged cousin," where there is a picture of Bellatrix over her name.
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: After bandaged Arthur arrives at Grimmauld Place, in the shot facing the twins, Molly hands the twin on the left the gift with blue wrapping and the twin on the right gets the orange wrapping. Next shot facing Arthur, as Molly pulls her hands away from the gifts, each twin actually has the opposite color than the previous shot and it is at this point they switch gifts onscreen.
Revealing mistake: In the Department of Mysteries, when Lucius removes his mask Bellatrix stands behind him, but her entire body has been digitally blackened. Additionally, there are two distinct shots in which there is a digital shape highlighting Bellatrix's face/upper body that moves as she moves. The first occurs when she remarks, "He knows how to play," then after Lucius begins to say, "Prophecies can only be retrieved," when he continues with the words, "by those about whom" it occurs once again in the close-up.
Continuity mistake: When Dudley is lying on the ground, with the Dementor hovering over him doing his thing, notice the wall behind Dudley with huge white graffiti. First he is lying past that specific graffiti, closer to the entrance/exit, then he is lying 4-5 ft away, on the other side of that graffiti.