Revealing mistake: After the lawnmower flips over, the camera angle from behind it shows the steering mechanism that's attached to the front wheels would not be on a normal one.
Continuity mistake: In the beginning of the movie, watch Kevin Bacon's hair when they're driving in the truck. In some camera shots, his hair is blowing in the wind. In others, his hair is not moving at all. This happens several times.
Continuity mistake: The green fuzzy dice in the guys' truck change over and over. Sometimes they're stuck up behind the rear view mirror, sometimes they're hanging down. Granted, they could fall and the guys could stick them back up again, but in one scene when they're behind the mirror, the faces of the dice that are visible from the front change.
Revealing mistake: When the last explosion happens after Val tosses it, what looks like detonating wire coming off to the left is visible for a few frames.
Continuity mistake: When all pole vault, Fred is holding the pole in his right hand palm up. Then he goes 1/2 the distance and returns to the start but his right hand is palm down. (01:00:00)
Continuity mistake: When Val is climbing up the electrical tower to get Edgar down, at first the sky is blue, then suddenly it's overcast. (00:13:40)
Continuity mistake: After Burt reports on the radio that he has killed the graboid in his basement, there are shots of the towns people celebrating. The sky behind all of them is different, specifically the mother and her daughter. In other shots there is blue sky. In these shots there are darker clouds.
Continuity mistake: In the scene where Walter is being attacked by the Grabbloid, the blood coming from his nostrils, and blood on his upper lip, changes between the first and second shot. (00:51:20)
Continuity mistake: When Val, Earl and Rhonda are on the rock and decide to pole vault off, Rhonda's backpack is flat when she scrambles down the rock, but appears full once she reaches the bottom. (00:40:20)
Revealing mistake: When Dr. Jim and his wife are backing away from where the generator was, you can see the ground behind them is flat. When it cuts, there is suddenly a pile of sand on the ground. This pile is hiding the hole that Jim falls into. (00:23:35)
Chosen answer: No, there's nothing illegal about any of the weapons they owned. Some of them may have required a Class 3 firearms license, but even this is not difficult (just expensive) for a law-abiding citizen to obtain.
Grumpy Scot