There are at least 2 helicopters used to depict the helicopter used by Mr. Wiley and Mr. Vincent; one of the helicopters has pads on the struts.
As David and Tom are sneaking into the alien encampment, Tom knocks out a sentry. In the long shot, the stuntman lying on the ground is visibly holding his head up off of the dusty ground.
The Invader drops the scientist's St Christopher medal that contains the homing device onto the floor. His intention is to stomp upon it with the heel of his shoe to crush it. This will be an impossible task; the close-up of the actor's foot reveals that he is wearing a soft rubber soled canvas shoe attempting to smash a small object constructed of metal.