Goofs

When the three astronauts are preparing to board the rocket they are in their spacesuits and carrying their portable oxygen units, but their face masks are open. Since the purpose of the tanks they are carrying is to acclimate them to a pure oxygen environment, the spacesuits should be completely sealed.

A Saturn V rocket cannot be launched from within the spacecraft itself; the launch command must be issued from the ground station.

Astronauts would not be allowed to be out roaming around on their own only eight (8) days before a launch.

The armed guard at the beach is "one of them", yet the actor, who isn't wearing concealing gloves, doesn't hold his right hand (facing the camera) with the gap between his ring finger and his pinkie.

Correction: It's already been well established that not all aliens suffer from this "crooked finger".

The door to the command module that the impostor seals after him is noticeably flimsy and wobbles.