When Denny sees the figure run across the road, he stops his truck and gets out to look around. Frightened, he gets back in and starts the engine. After the figure appears and puts the cow's head on his hood then disappears, he starts the (already running) engine again before driving away.
It is stated that the special rifle is capable of firing a bullet at Mach 20 (approximately 22,000 feet per second). This is not possible. Such a velocity would be well more than four times faster than the modern (2010) rifle/bullet combination with the highest muzzle velocity (the 5mm/35 SMc), which has a muzzle velocity of about 4,800 feet per second.
It is stated that the special rifle will sometimes spit out a three round burst when it gets hot, and that as a semi-automatic it is still legal. This is wrong. The BATF ruled that a weapon which fires multiple times for one pull of the trigger regardless of cause is classified as an automatic weapon and requires a special license.
When they are out hunting for the demon at night, a character leans down to pick up Earl's safety vest. The rope used to lift the character and pull him backwards is clearly visible in two shots: once behind his back and once between his legs.