Character mistake: In the scene just before Harry and Cho kiss in the Room of Requirement, there is a clear Daily Prophet headline to the right of the mirror - 'Mystery Disappearences' a mis-spelling of the word disappearances.
Plot hole: Harry can now see the thestral creatures because he witnessed Cedric Diggory's death in "Goblet of Fire." However, he should have been able to see the thestrals after Professor Quirrell died from Harry's touch in "Philosopher's Stone". Quirrell crumbles into dust (dying) while Harry is still conscious, watching, and fully aware of what he is seeing, thus allowing it to fully sink into his consciousness. Harry faints "after" Quirrell is dead and as Voldemort's spirit passes through his body. Nor does it only have to be a friend or loved one who dies before the thestrals can be seen - it can be anyone, including an enemy like Quirrell. There only has to be an emotional response to the death, which can be love, hate, fear, shock, etc. (In the book, Harry did not see Quirrell die, but that's irrelevant to the film).
Suggested correction: JK Rowling has said that in order to be able to see thestrals the death has to "sink in" psychologically with the person. Which was her explanation for why he couldn't see thestrals even though he witnessed his mother's death. Given Harry's age and the way Quirrell died, it is possible that Quirrell's death didn't have enough of a psychological impact on him to "sink in." After I first personally witnessed a violent death, I didn't feel the impact until a few years later. It can take time.
Continuity mistake: After Harry declares "If Voldemort's raising an army, I want to fight", it cuts to him sleeping in bed, and the blanket's position differs between shots.
Continuity mistake: During the movie, Harry's scar noticeably moves on his forehead and is not always in the same place.
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.
Continuity mistake: During the OWLs examination in the great hall, the large pendulum that swings on the large stage behind Dolores Umbridge; during the scene the swings do not always match up from shot to shot.
Visible crew/equipment: When Harry and Dudley get attacked by the Dementors near the start of the movie, when Harry runs over to Dudley after he casts a spell on the Dementor, you can see the microphone wire underneath Dudley's shirt whilst he is lying on the floor.
Continuity mistake: When Harry is narrating a letter to Padfoot, he sees a Thestral flying in the far distance, and it disappears behind a mountain. He hikes (presumably) all the way to where he thinks the Thestral went, and he finds Luna Lovegood in a clearing with a few Thestrals. For one thing, the Thestral that disappeared behind the mountain was MILES away, and it would have taken Harry a couple of hours, at least, to hike that distance through mountainous terrain. But, when he arrives at the clearing with the Thestrals, the daylight hasn't changed at all (should have been noticeably different lighting). Once he arrives there, he sees that even the largest Thestral is no bigger than a horse. But, when he first saw the Thestral disappear behind the mountain, at that distance the Thestral was about the size of a commercial jet airliner.
Revealing mistake: Just as Dumbledore is about to introduce Umbridge the shot cuts to Umbridge. Look at the candle to the right of Snape; you can see a string holding on the right hand side of the candle holding it up. You can also see the strings on all three of the candles.
Continuity mistake: When Harry brings Dudley home, just before he opens the door Petunia asks, "Diddykins, is that you?" and the plant fronds (beside the door) block the bottom of the photograph behind it, but in the next close-up the fronds are lower so the photo is more visible.
