Other mistake: As Dr. Challis is sneaking to the console to activate the flashing pumpkin, the digital clock on the wall behind him shows 11:35. It should still be before 9:00.
Other mistake: When Marge Gutman accidentally triggers the laser chip in the Silver Shamrock tag, you can see the laser "tethering" to her mouth when she pulls back, instead of shooting out in a straight line. This movie never once hints at the existence of target-locking or mouth-seeking lasers.
Other mistake: Why is it that Marge only discovers the microchip in the Silver Shamrock badge when it drops on the floor in the motel room? Surely she would have noticed it when her son would have thrown the mask against the wall, and it dropped off? Any normal person would have looked at it to see how it would have attached to the mask.
Answer: He was trying to perform a human sacrifice. He mentioned to the doctor when he was tied up that three thousand years ago, there was a massive sacrifice where the world flowed with the blood of children and animals. Now the planets were aligned again and it was time for a new one in order for the people of his craft to control their environment.
Zorz