Revealing mistake: When Jack Sparrow is chased in London, being on the carriage and looking back to his chasers, you can see the broad tire tracks of a modern vehicle in the mud. (00:16:55)
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)
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.
Revealing mistake: You can see the mermaid's underwear after she is given only a shirt by Philip. It appears to have red in it, which can be seen through the white shirt given to her.
Chosen answer: She most likely went to stop the ritual, she probably didn't want her tear to be used by the likes of Blackbeard. She knew that Jack would use it wisely.