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.
Picture added & edited by Super Grover
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