When Val, Earl and Rhonda are on the rock the morning after they first discover the graboids, when Rhonda puts on her backpack before she gets the pipe to use as a pole to vault off the rock, it is empty, (flat). When she is shown from the back, leaning off the rock to pick up the pole, the backpack is full.
When Val, Rhonda and Earl flee the second graboid by climbing onto a rock, Val is wearing cowboy boots. When the camera cuts to the rock and Val climbs to the top, he's wearing leather workboots. After they wake up the next day and Val slides down the rock to see if the graboid is still there, he is again wearing cowboy boots.
When Earl goes to where Rhonda found the pole-vaulting sticks, two of them are conveniently placed right next to the rock, but when Rhonda first took a stick, they were in a random place farther from the rock. Further, the poles grow in length from the time they are picked up by Earl until the long shot of them being used.
During the scene where Earl and Val play paper/rock/scissors on the rooftop to see who goes get the tractor, the sky keeps changing from cloudy to cloudless. This happens quite a bit in other scenes too.
When Burt and Heather are being attacked in their rec room and Heather is grabbing another weapon off the wall there are two pistols hanging near her right shoulder. While the ground is shaking, the top one can be seen falling off it's pegs but when the scene flashes back to Heather only one or two seconds later, it's back on the pegs.
When the tractor 'decoy' is deployed downtown, two separate graboids are clearly observed giving chase (Nancy even stating "This one's going too!"), while a third graboid is still stalking Burt & Heather's compound a considerable distance away, well outside of town. However, by that point, it was already established that there were only two of the creatures remaining in the valley.
Near the end, when the three run for the cliff, when Valentine McKee (Kevin Bacon) first starts moving you can see the platform that he is standing on that is buried under the sand (probably covering the worm mechanism that surfaces just beside him) shift as he moves. It's just the size of a sheet of plywood, and the moving dust shows its outline.
When Earl and Val are trying to get away from the construction crew site after the road becomes blocked and he gets "hung up" on the rocks you can see the reflection of a man wearing red shorts and a white shirt in the truck's rear window.
After removing the piece of graboid from the back bumper of Val and Earl's truck, the townspeople take pictures with it. Rigormortis usually sets in for any living creature within hours after death. However, when Melvin comes out of the shop screaming, he has the graboid piece around his neck and it is easily movable.
Many times during the movie someone will make a noise close to the ground and nothing tries to eat them. However, there are only four of the creatures and it is a big valley. Just because someone is making noise does not mean a creature will immediately appear and try to eat them. They may be off eating Miguel's cattle or in some other location in the valley too far to get there in time.
When they are on the roof talking on the CB radio, they manage to use a base station unit without a power cord connection. Many CB radio bases are powered by batteries. Apparently, this is one of them. Even if that particular model was designed to be AC-powered, Walter could later have had it customized for DC power.
When old Fred is tending his garden, there is a blue car parked between the garden and his shack. Later, when Val and Earl arrive and look for Fred, the car is gone. However, graboids are notorious for pulling cars under the ground, so this disappearance should not come as a surprise.
When Burt and Heather are being attacked in their basement by one of the creatures, he breaks the glass of the display case and loads an extremely powerful 8-gauge "elephant" gun. Burt then takes aim and fires twice, eventually killing the monster. The problem is that when preparing to fire, he puts the butt of the gun UNDER his arm and not against his shoulder where it has to be in order to fire effectively. Had that been a real gun of such a size and caliber, the recoil would have, at the very least, launched the gun backwards under his arm, and in a worse case scenario, either had the barrel of the gun strike him in the face because of the downward angle of the butt or had his wrist broken as the gun would have moved faster than the physical reaction needed in order to get his finger out of the finger guard.
When the guys whip the truck around in a 180 degree turn to
return to the guy on the power pole, you can see the roofs of a housing development on the horizon, as opposed to being in the "middle of nowhere."
In the final showdown between Val and the final worm, when Val bails before the worm bursts through the ground to surprise Earl making him run, you can notice that the ground that the actors are standing on is a fake.