At the humble desert ranch with the horse corral of the Mexican Americans, a roadway passes right in front of it - where the rancher's pick-up truck is parked. During the closing, that exterior view is shown from inside the house through the front door: that view is of an obviously painted backdrop of desert with cactus beyond a rickety pole railing, and no road next to the house, nor the edge of the attached high wood corral. In the next exterior shot of the ranch - the road, corral, and solid ground to roof pole supports are seen - not a rickety railing and desert sand with cactus.
During a foot chase through the rocks, Vincent takes a rest, setting his canteen down, but doesn't pick it up when he runs away. A few moments later he's holding the canteen and uses it to smack the face of an approaching alien.
Vikki's name appears with different spellings in different contexts. A sign outside the club where she works spells it "Vickie" while it is spelled "Vikki" on an interior sign. (One mitigating factor might be that the setting is bilingual with both Spanish- and English-speaking customers, but this would not adequately explain the discrepancies.)
Vincent walks toward the Hotel Popular and the cars that are parked outside of the building. Moments later, he is saved by Evans from being run down, but the cars outside the hotel are not the same.
The alien ships seen in the show are all "reasonably" sized. As the alien ship takes off from behind the hill, the perspective is wrong as the ship appears to be immense, appearing to be far larger than objects in the foreground and on the near side of the hill.
When David and Vikki are driving out to the desert, they are driving at highway speeds in an open vehicle, yet Vikki's long hair stays perfectly still.
When David looks up at the fire in the kitchen his stunt double is seen in the kitchen on fire before Vincent ever runs into the kitchen.
During a fight with an alien, the alien drops his weapon. Mr. Vincent could have used this as evidence of their existence.
David is in the sights of the disintegrator ray yet the aliens do not fire. The aliens have had no problem sacrificing themselves and other aliens, so killing Vikki as collateral damage to get Vincent would not be a problem. Instead of killing Vincent, they disintegrate the Jeep.
When the first alien catches Vincent in the rocks he walks toward him rather than shooting him, giving Vincent the opportunity to knock the weapon from his hand and engage in a fist fight. It is nonsensical since the alien does not need to get close to Vincent to shoot him and there is no reason to hesitate since their goal is to kill Vincent.
Any one of the aliens could shoot the barrel Vincent was hiding behind, then shoot him once it disintegrates, but they run out into the open so Vincent can shoot them dead instead.
The spaceship is supposedly broken and unable to fly yet it is miraculously repaired just in time so that it can fly away at the exact moment Vincent kills the last alien.
David Vincent is always trying to prove that the aliens exist. He knocks the disintegrator gun from the hand of one of them in the fight, but doesn't then think to retrieve it afterwards as easily convincing evidence since nothing like it exists on Earth.