When John, Ellen, and Pete return to the airfield in the helicopter, they have a brief conversation with rotor noise in the background; yet neither the main rotor shaft (behind them) nor the tail rotor are moving.
Perry, wearing a space cadet suit and helmet, opens the hatch over his face. The point of view quickly switches to Ellen and the open door, as Perry walks through the doorway, but now the hatch over Perry's face is closed.
Artistic license and suspension of disbelief notwithstanding, there is a scientific oversight worth noting: the alien's POV is in 3D, despite the fact that it's a cyclops. The otherworldly visitor has one eyeball and one pupil, when it's necessary to have at least two in order to perceive stereoscopic vision.
After the spaceship has crashed, when they look through the telescope to see the crash site the image is the correct way up but since this is an Alvin Clark refractor telescope, the image should be upside down.
When Putnam and Ellen land back at the airport in the helicopter, Putnam is holding his pipe in his mouth upside down (bowl facing down). Pipe smokers sometimes have the bowl upside down, especially when it's raining. They tamp the tobacco tightly, light it, and then turn it upside down to keep the rain out of the bowl. Some pipe smokers will have them inverted even when it's not raining, possibly as an affectation, or simply habit.
When the aliens assume the forms of all the people they've kidnapped, they're always wearing the same clothes their victims had been wearing when they were taken - with two exceptions: Putnam and Ellen. In their cases, the aliens raided their homes and stole clothes for their "duplicates". First: how did the aliens end up in the same clothes worn by the other people? Did all these people have two identical sets of clothing lying around their houses? Second: why steal different clothing for the Ellen and Putnam clones? Third: at least in the case of Ellen, why did they empty out her entire wardrobe instead of taking just one dress? Fourth: if, under a different but unstated hypothesis, the aliens only appeared to be dressed the same as the others - i.e., through morphing or some other illusion - then why would they need to steal anybody's clothes?
When Richard Carlson and Barbara Rush blow out three candles, there's another half-second delay before the light in the room actually goes out. Clearly, the off-stage light, that was supposed to go out at the same time the candles were blown out, was turned off just a second too late.