1:At the beginning of the scene near the end of the movie with Lucius Malfoy fuming at Dubledore in his office, Malfoy's hair is fanned back behind his shoulders. The lighting in the room illuminates the back of his neck, where you can see his real, short brown hair.
2: When Lockhart falls down the hole into the Chamber of Secrets, we hear him hit the ground a second or two later. When Ron and Harry jump down, not only do they take longer to get down, they also slide down the pipe, rather than fall straight down.
3: In the Quidditch scene, Harry breaks his right arm, but as he sits up after he falls off his broom, he leans directly on it.
4:When Myrtle is flooding the girls' bathroom, the tap that later doesn't work (the entrance to the chamber) is happily churning out water - it's on the side nearest the door, so we see it clearly both times.
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