The Phoenix Saga (1): Sacrifice - S3-E3
Continuity mistake: When Cyclops is saying "They'll be safe in the isolation chamber " he and Jean Grey are standing near the lockers on the left side of the screen, four floor tiles away from the door. When the shot changes, the two of them have suddenly moved to the right, in line with the door, and only two floor tiles away from it. The floor tiles themselves are also now drastically smaller. (00:08:32)