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. (00:34:00 - 01:09:35)
Continuity mistake: When Xenophilius draws the Deathly Hallows symbol, he draws it so that the bottom of the line touches the bottom of the circle. The shot cuts away and back again when he draws the triangle, but the line is no longer touching the bottom of the circle. (01:49:40)
Continuity mistake: At the snake exhibit, while Harry speaks with the Python the snake's body lies curled over a long wood log and a large rock. In the shot from outside the Python's exhibit, after the glass pane disappears Dudley falls over the rail and the snake is gone, not positioned differently, but altogether gone. In the next overhead shot inside the Python's exhibit, while Dudley falls the snake is back lying on the log and rock, in the same position as in the previous shots. (00:07:10)
Continuity mistake: When Malfoy and his friends bother Ron and Hermione, Harry shows up and messes with the three boys. He grabs hold of Malfoy's feet, lifts them in the air and pulls him across the snow. In the first close-up of the soles of Malfoy's boots, not a speck of snow is on them, though he was just walking in it. Then in the next close-up, his feet still in the air, the soles are now covered in snow. (01:02:45)
Continuity mistake: In the dormitory, when lovestruck Ron is going gaga over Romilda, he is wearing a brown sleeveless tee shirt and its tan trim has many holes. When Harry brings Ron to Slughorn, Ron is still wearing that tee shirt, but now some of the holes are gone and the rest of the holes have changed position. (01:26:50)
Continuity mistake: During dinner at the Goldsteins', the dishes are empty from one shot to another.