Other mistake: In the Forbidden Forest, after Grawp has put Hermione down after grabbing her, he turns round and bends down to pick something up. When the shot turns to Ron, Hermione and Harry they are still looking up at Grawp even though he's bending down. Their position doesn't move. Ron even looks Grawp up and down, even though he's still bent over with his back to the students. (01:27:10)
Continuity mistake: When Snape grabs Harry by the collar of his shirt after Harry has just seen Snape's memories, the position of Snape's thumb changes between shots. (01:29:50)
Continuity mistake: When Hermione and Ron meet Harry at Grimmauld Place, Hermione's hair changes from in front of her shoulder to behind it and back to the front. (00:13:00)
Continuity mistake: During Harry's first detention, before he starts writing and while Umbridge is still talking, he's sitting at the desk. He's holding the quill in his right hand, and his left hand and arm are resting on the desk. The shot switches to another angle, and his left arm is immediately off the desk. Umbridge is still talking, so there is no time lapse between the two shots. (00:37:10)
Continuity mistake: During the fifth-year O.W.L. exam in the Great Hall, Umbridge is sitting at the head of the room in front of the swinging pendulum. When the Weasley Wildfire Whizbangs go off in the Entrance Hall, Umbridge leaves her seat. The distance between her and the Great Hall door changes between camera shots behind and in front of her. (01:31:25)
Continuity mistake: The Weasley boys interrupt the exams with their fireworks display. When they fly off, all of the proclamations in their wood frames fall to the floor. The proclamations above the door fall in front of the doorway at Dolores Umbridge's feet. When all of the students run out of the room, all of the proclamations that were in front of the doorway have instantly disappeared.
Visible crew/equipment: When Harry is walking out of the Dursley's house with Moody and the others, as they walk across the front of the house, just after Kingsley says, "Dumbledore has persuaded the minister to," on the left-hand side ground floor window the reflection of the moving reflector screen is visible. (00:08:40)
Revealing mistake: When Harry is going down to Hagrid's hut and discovers he isn't home, he looks over the forest discovering the thestrals. The camera moves back so Harry is also visible in the shot, but as he becomes visible, just shortly at his head to the left some of his head goes missing showing that it is a chroma-keyed background / blue or green screen. (00:40:50)