Revealing mistake: When the boat is supposedly surrounded by a swarm of flies, they're really just relatively few flies trapped between two panes of glass in front of the camera lens. Evidence: whenever the camera moves, the "swarm" moves with it against the background, and you can even see some of the insects walking on the surface of the glass.
Revealing mistake: When the torpedo is fired at a Japanese ship, the underwater shot of the torpedo cruising reveals it being pulled by a long wire. It is also apparent that it was filmed in a tank with the reflection of the sun on the tank wall.