The Phoenix Saga (2): The Dark Shroud - S3-E4
Continuity mistake: When Xavier and Cyclops begin arguing in the hallway, the door and doorway to Jean's hospital room are visible over and behind Xavier's right shoulder, and a chart is a few feet to the right of the door. A few shots later when Cyclops is telling Xavier that he failed to warn everyone, the chart is still visible, but now the door is missing, with the wall now appearing to be longer. (00:06:40)