In the opening sequence, the approaching aircraft is described as a U-2, a high altitude reconnaissance aircraft with a narrow fuselage and a wide straight wing span. However, the aircraft shown is a Sabre Jet F86 with distinct swept back wings.
When Admiral Nelson rigs the electromagnet using the mini sub's batteries in order to open the airlock door, a humming sound is heard. Since battery output is direct current, there would be no sound at all.
When the Lieutenant attempts to shoot the General, his weapon will not fire because of the beam. In a later gun battle, the General fires an identical weapon while standing close to the machine generating the beam.