Revealing mistake: When the collector gets into his car, he tries to close the car window. Robert puts his hand through the window. The collector manages to close it well enough to trap Robert's hand and takes off in his car. From a camera view outside the car, you can see the window isn't closed enough for Robert's hand to be trapped.
Revealing mistake: When Barry has shot the alien in the head at the beach the camera is filming the aliens feet as the blood starts dripping. Then the camera start moving up and when the camera reach the alien's waist level you can see a sudden cut. This is done so the fake alien puppy with half its head gone can be used.