An Extra Passenger
There are several times throughout the film where crewmen are visible in the shots, but this is the most glaring mistake that was overlooked. As Jack is greeted in first class, we travel through the ship, apparently seeing everything through Jack’s eyes as we move forward.
But approaching the glass doors, we can suddenly see that we’re not in Jack’s shoes at all. It’s a cameraman who’s lugging a big camera who’s visible in the reflection, and was never edited out.