Other mistake: After the wreck falls from the chopper it starts moving forward with broken sails - with the anchor set solid in the reef.
Revealing mistake: During the scene with the ships hanging from the helicopters, Nate decides to fire a cannonball at the other helicopter as it's coming towards them. A standard cannonball shot from a 36 pound cannon that the Spanish used on their ships weighs 16.5 kilograms (the weight of a large full water dispenser bottle) but Nate tosses it around like it weighs nothing.
Continuity mistake: When Nathan gets out of the trunk of the red car in the cargo plane he is not wearing his ring necklace for a couple of scenes. Then it appears.
Continuity mistake: When Nathan exits the manhole in Barcelona his fringe changes style depending on the shot.
Continuity mistake: When Nathan prepares the Negroni the mixing glass is either parallel to the napkin or to the left of it. This changes back and forth.
Continuity mistake: When Nathan arrives late to work he extends his right arm to take his jacket off and then lowers it. From the opposite angle his arm is extended, and he lowers it again.
Answer: Jo and her mercenaries were at the wrong island, It's when they saw Drake go past them in a speed boat, and not to the island they were on, that's when they realised they had been tricked.