Revealing mistake: During the car chase scene, if you watch the speedometer on the car, it never rises above zero. That, and the obviously poor image quality of the "road" in front of them, shows that they were using projected film in front of the car.
Revealing mistake: The flashlight that the drillman used when the lights went out around the spaceship didn't match the light from the spot used by the film crew.
Revealing mistake: In the scenes inside the space capsule, as Roy goes through the sequence, the wires holding him up in the floating sequences, and the crackers when trying to eat, are visible.