A camera reflection
Despite there being other moments in the movie where crew persons can be seen in the scenes, this is the most obvious error that the director missed. We board the ship and are welcomed in first class. It seems like we are experiencing the world through Jack’s eyes the entire time.
But as we go closer to the glass doors, it becomes apparent that we are not at all in Jack’s shoes. The glass reflection shows a camera that was never cropped out.
