Deliberate mistake: The four main "good guy" characters are stuck on an island with people who are trying to kill them. Steven Baldwin's character brings a pistol and takes a machine gun from one of the bad guys. Several times they overcome other armed men, but after defeating them, never bother to take the bad guys' guns. At one point, two characters argue over who gets to carry the pistol, yet when they knock out a man armed with a rifle, they leave it with him and just walk away.
Deliberate mistake: The map and overlay that lead everyone to the island are supposed to be over 500 years old. Yet they're in amazingly good shape and hold up very well considering how roughly they're handled.