The mark on Billy's chest appears and disappears intermittently: Kathy asks about it when she and Billy are lying next to the van, then when they go to the pool party, it isn't there anymore. Later, Billy shows the mark to the police, and it's clearly visible as a silver "spot", but then when he goes on his final rampage all over the town, there are several occasions when we can see that it's gone again.
When Billy first visits Kathy's house, he puts on his shirt just after he gets out of his van. In the next shot, as he gets to the front yard, he is carrying his shirt, which he puts on as he enters the yard.
When Billy first discovers the laser gun and pendant in the desert, he puts the pendant around his neck. But in the next shot when he fires the gun for the first time, he's holding the pendant in his left hand.
When Billy first finds the laser and figures out how to shoot it, he puts the amulet around his neck and continues to shoot. But in the scenes where he is shooting after he dons the amulet, he is still holding it in his hand.
The back doors on Billy's van are closed when he's pulling out of his driveway in one shot, but when the cops pull a U-turn and question him, the shots through the windshield of the van reveal that one of the van's back doors is open.
When the the Star Wars sign that is blown up first visible, there is no car ahead on the road. The the laser is then pointed out the window, and there is a car about 100 yards ahead, which starts braking. The same scene appears at a slightly different angle, and there is no car there, and the sign is blown up.