Other mistake: If the scene in which Jack Aubrey gives the order "hard a larboard" [meaning the opposite of starboard] was indeed a rudder order (ie. turn the rudder in the direction commanded, which turns the ship the other way, in this case to starboard) the face of the compass wouldn't have rotated clockwise. The compass shows a southerly course, rotates clockwise, and eases to show an easterly course, which is a 90 degree turn to port.

Master and Commander: The Far Side of the World (2003)
1 other mistake
Directed by: Peter Weir
Starring: Russell Crowe, Paul Bettany, James D'Arcy

Revealing mistake: After a close-up of the drawing of one-armed Lord Nelson, Killick opens the lid of the steaming food for Jack in a close-up, just before Killick says, "Oh, here we go again. Scrape, scrape, screech, screech." Later, after Pullings takes command of Acheron, Jack tunes up his violin and Killick says, "Here we go again," in another close-up of the steaming food for Jack. Every detail of the food shown in both close-ups is exactly the same - obviously shot at the same time. (00:26:25 - 02:05:20)
Capt. Jack Aubrey: To wives and sweethearts - may they never meet.
Trivia: Paul Bettany describes the scenes in which he collects insects: "I don't like insects. They tend to scamper and scuttle, so it was difficult." He also practiced for his self-operating scene by sitting "at home and [trying] it with a butter knife in front of the mirror."
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Chosen answer: The flag is a signal flag, and is probably meant to signal that the ship is a whaler or engaged in fishing operations. Such flags were part of an international code, which, with some modification, is still in use today. (The flag is similar to the modern-day "Foxtrot".)