Chrille said:
There is one officer that is wrong, though, and it seems like a shame when everyone else are perfect. He appears briefly as an extra without lines in Vader's first scene. As Ozzel and Piett are argueing and Vader is making his way over to join them, the camera pans down to the trench and an officer standing looking up at Vader. He is wearing the rank insignia of a navy captain, even though Piett is captain of the ship at that time.

(Desperate rationalization follows)

No doubt that officer is indeed a Navy Captain, hastily promoted from Captain of his own vessel to serve on Executor (the Imprial flagship) due to the power vacuum in the chain of command created by Vader's habit of strangling everyone. If I recall my boyscout days, an officer with the rank of 'Captain' serving on a ship he is not the captain of, would be addressed as 'Commodore' (I think).

(An army or marine captain aboard a ship would be addressed as 'Major,' a rank higher than they hold, to avoid confusion with the ship captain as well).