Revealing mistake: During the gemstone smuggler robbery, when Verbal kicks open the van's rear doors, McManus begins to use a sledgehammer on the NY police cruiser's windshield, and behind him we see two very tall palm trees―in NYC. This scene was filmed in Los Angeles. Note the police cruiser has number 2485 at the airport and turning corners with the van tailing (filmed in NYC). As the van hits from behind it's 6423, and the stunt vehicle set on fire is 6426 (filmed in LA).
Revealing mistake: Towards the end of the movie, Hockney (Kevin Pollak) 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: While the gemstone smuggler is riding in the back seat of NY's Finest Taxi Service, in his close-ups note the vehicle's chrome button door lock, the garment hook over the window, etc. When the police cruiser makes a short stop, now in the smuggler's close-ups the door lock's chrome button cover is gone so only the inner pin remains, the garment hook has been removed, etc. (00:32:40)
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.