Factual error: Captain Barbossa's ship has a modern Union Jack flying on the stern. The film is set in the reign of George II (1727 - 1760). The flag shown includes the cross of St Patrick, although Ireland did not become part of the United Kingdom until 1801, and the Union Jack at the time of the film would have comprised only the crosses of St George (England) and St Andrew (Scotland).
Factual error: The soldiers chasing Sparrow in London have red cockades on their hats. They should be wearing Hanoverian black cockades, as George II was the second king of the House of Hanover.
Revealing mistake: When Blackbeard threatens Jack with a pistol, he is heard cocking it, but when he holds it up to the camera we can see that it is only on half-cock. A pistol cannot be fired in the half-cock position, nor can it be loaded in the fired position, implying that either it is not loaded, or a technical error has been made by the film makers.
Visible crew/equipment: You can see mermaid's underwear after she is given only a shirt by Philip. Appears to have red in it which can be seen through the white shirt given to her.
Continuity mistake: When Jack and Gibbs are coming out of the carriage by the palace, a soldier tosses a little bag with money to the carriage driver. In the next shot, you see the soldier tossing something to the driver again.
Factual error: After the mermaids scene, when Blackbeard, Jack, and the others start walking through the next island to find the silver chalices, two distinct bird calls can be heard. Both are native to Australia - they are the Magpie Lark, also known as the Peewee, and the other is the Willie Wagtail. Both birds are not found anywhere remotely near the Caribbean.
Revealing mistake: Just before he sees the droplet of water moving upwards, Jack moves three leaves to get through, but the top-right one is just held in his hand and you can see the cut stem of it. (01:42:20)