Continuity mistake: When the police car rolls out of the car wash, both passenger and driver's doors are open. When the shot changes to the deputy holding the rifle, the driver's door is closed. In the next shot both doors are open again.
Continuity mistake: When the principal stabs the 3rd victim (male) on the gurney, we see that the tongs on the pitchfork extend almost their full length underneath the bed. However as we see up top, the pitchfork couldn't have been that deep given it had the height of the man's chest to go through as well.
Continuity mistake: David is stabbed in the right hand in the baby's room. Throughout the rest of the movie his hand is either not bandaged anymore, or is on the wrong hand, such as seen when David is interrogating the driver of the vehicle Russel throws the spike strips at.
Continuity mistake: During the baseball game scene with Rory, the deputy is holding his firearm in his right hand with his holster on his right hip. When it cuts to the overhead view it's a reversed shot, notice for instance the deputy holds his weapon in his left hand and holsters it on his left hip.
Continuity mistake: When the sheriff is fighting the crazy underneath the truck and covers him in oil, the crazy pulls out a gun. When the sheriff initially ignites the oil the gun is missing, only to reappear when the crazy is completely on fire.
Continuity mistake: At the end when we zoom up from the Sheriff and his wife, the "spy satellite" view zooms significantly back but we don't see any of the telltale evidence of the recent massive explosion to the south of them that was seen in the distance while they were walking through the field.
Continuity mistake: As the 3 hunters investigate the parachute they pull on the cord and the dead pilot pops up. The middle hunter then falls into the water and drops his rifle in the water. The next shot is the overhead shot showing that he still has the rifle in his hands. (00:17:55)
Answer: The third time is by not shooting him right there.