Revealing mistake: Towards the end of the movie, Hockney (Kevin Pollack) looks into a van and discovers it's full of cash. He is then shot from behind and blood splatters all over the money. When his chest explodes, you can see the plastic of the bag the blood was in. (01:18:10)
Revealing mistake: When the police car is set on fire you see from the top that the flames start at three different spots, hood, roof, and trunk. Then, when the three vans drive off there's a shot from inside the police car where you don't see any fire. (00:32:45)
Revealing mistake: In the flashback, when the boy has his throat slit, there's no blood or wound.
Answer: He never did. How they got on the boat and took out the Argentinians could have actually happened in a million different ways. For all we know, verbal went on the boat with them, and then killed them, which is more likely as it is an extra gun. The point of the film is to show how much of a mystery Keyser Soze is, that even after all of that, no one knows who he is. For example, he tells the story as if Soze is Hungarian and killed his wife and kids, but we know this isn't true, because Kint is Soze and he's American. The point is, he's telling so many lies and making up all these stories, so it's got to a point that they don't even know his real nationality. The only reason we though he was Hungarian throughout the film is because Kint told us, but we know his story was a hoax.