Why doesn't David kill the two alien captors he knocks out? This would not only eliminate the threat, but any loss of personnel would hinder the alien operation.
For a security man and former cop Tom makes a serious mistake. After knocking out the armed alien patrolling outside the mine, he fails to disarm the alien, making him a renewed threat when he awakes, and also passing up evidence in the form of the "ray gun" of the alien presence on Earth.
Tom leads David up on some rocks to show him the abandoned mine. It is strange that David didn't notice the mine earlier, since he was walking from that direction at the beginning of the act.
David is in close physical contact with Millington, yet he doesn't notice the bulky alien gun Millington produces a moment later when he attempts to shoot him.
When David is picked up at the airport, a sign reads "Maricopa Airport," yet in the establishing shot of passengers exiting a plane a sign in the background reads "Welcome to San Diego."