Continuity mistake: When Val and Earl are being chased by the graboid in the first shot it appears that the graboid is going perpendicular to the fence (you can see this because of the wires that cross it). But in the next shot the graboid appears to be going parallel to the fence. (00:33:20)
Continuity mistake: When the snakeoid is chasing Val and Earl after they lose their horses, they run along a fence and the monster causes all the fence posts to fall over backwards. But shortly after, all the fence posts are seen to be upright, although some are tilted a little to the right.
Factual error: When Val is talking to Burt on the CB radio and desperately trying to tell him to get out of his basement, Burt is using a handheld CB. After Val's very last transmission, Burt is now holding a rifle, not the CB. When the Graboid crashes through Burt's basement wall, we hear him say over Val's CB, "Jesus Chr..." his transmission was cut off. First of all, Burt wasn't holding the CB so he wasn't transmitting at the time. Second, there are portable CBs that you can lock the PTT button for extended transmissions (I owned one), but that wasn't the case here. Finally, even if by some chance Burt accidentally did lock the PTT button before putting it down, it wouldn't have cut off. The Graboid broke through away from where Burt was standing and talking and couldn't affect the CB. It was done for dramatic effect of the scene. (00:58:30)