Continuity mistake: In the big fight scene, Roddy Piper and Keith David are wrestling and struggling about and land so the black guy's pants are getting soaking wet. In the very next shot as the fight continues, his pants are dry. (00:58:55)
Continuity mistake: When Nada reads the magazine at the book stand, he removes the glasses, you can see that there's a graphic on the right page but when he put them back on in the following shot, the page is different. (00:34:20)
Continuity mistake: During the big fight scene, Roddy Piper smashes the back window of a car with a piece of wood. In the very next shot, Keith David rushes Piper and continues to fight with him. Right before the shot ends, you can see the same car in the background, smashed window and all, now at least two cars over from where they are fighting. (00:59:30)
Continuity mistake: When Roddy Piper first puts on the sunglasses, he stops at a newsstand, where he sees his first alien. Watch as the alien guy pays for the magazine he's buying. The vendor gives him change twice, and even says different things to him each time he hands him the change: 1. "Here's your change sir." 2. "Here you are sir. Thank you." (00:34:55)
Continuity mistake: After Roddy Piper's character spends the night at the homeless camp, he walks towards the church wearing a toolbelt. He stops and watches the Street Preacher recite what is airing on television. As the shot switches back from the Preacher to Piper, his tool belt has vanished.
Continuity mistake: During the big fight scene between Piper and the black dude, they begin to grapple and roll over. Piper's belt becomes buckled and unbuckled between various shots. (00:58:35)
Continuity mistake: Rowdy Roddy Piper is in Holly's car trying to escape from the aliens after he attacks the bank, he is ducking down by the passenger's side front door lock. In one shot the door lock is up. In the next it is down. Then up, then down... (00:44:40)
Continuity mistake: When Nada and Frank are trying to get to the roof of the television building, at some point they shoot at two guards and a worker runs away. In the following shot, that same worker is closer to them. (01:26:25)
Continuity mistake: When Frank and Nada first start fighting in the alley, Frank throws his knapsack in the middle of the alley. During the fight it is moved next to a dumpster (as well as the cardboard box of sunglasses).
Continuity mistake: After leaving Frank at the construction site, Nada finds the box of shades in a garbage truck. However, there's only a few pairs, compared to when he hid the box, and it was full.
Continuity mistake: When Nada shoots the cops the pistol is a nickel plated .357. When he's in Holly's car the .357 is black.
Continuity mistake: Nada is wearing a green flannel the morning after he sees the preacher led into the church, but it's blue when he asks to borrow the binoculars just minutes later.
Answer: As part of the alien takeover, they're suppressing any resistance and discussion of the plot. They know there's a group, led by the preacher, who are onto the conspiracy and that they're somewhere in the camp. They destroy the camp to disburse them and prevent word from getting out.