An extra traveller
There are other instances in the film where crew members are visible in the frames, but this is the most egregious oversight. We travel through the ship as Jack is received in first class, seemingly viewing everything through Jack’s eyes as we continue onward. However, as we approach the glass doors, it becomes clear that we are not in Jack’s shoes at all. It’s a cameraman with a large camera who can be seen in the mirror and who was never edited out.