Other mistake: When Nate and Sully first meet Chloe in Barcelona, Chloe steals the cross from Nate. In none of the shots is Chloe's arm reaching anywhere. And the backpack never appears to be jostled. (00:33:45)
Other mistake: Sully climbs up the helicopter chain in the final scene with a bag full of gold. Assuming modest dimensions of 30x30x30 cm for his backpack, that means that he was carrying at least 40 kilograms of gold on his back, but he tosses it around like it's full of fluff. And when he hits Braddock in the face with it later, he's tossing 40 kilos of gold at high velocity at her face, but she shows up later without a scratch.
Other mistake: Sully jumps off the plane with the map but somehow Nate and Chloe end up with it.
Other mistake: There's actually no need for two people in order to get to the chamber with the huge urns. You could easily just pry open the grate by the fountain, rappel down and bust through the final door by some means (or have both of the keys).
Other mistake: After the wreck falls from the chopper it starts moving forward with broken sails - with the anchor set solid in the reef.
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.