Revealing mistake: When Jim is sitting in the back of the police car, after taking the two policemen hostage, John shoots at the two cops. When the cop on Jim's right gets shot and the window smashes, blood jets up from behind him - not from his neck. (00:44:30)
The-ImmortalContinuity mistake: When Jim eats at Nash's workplace, in one shot there is no napkin, knife or fork in front of Jim's plate. Then when Nash walks away it cuts to another angle, and now there is a knife and fork on a napkin. (00:28:00)
The-ImmortalContinuity mistake: When Jim stops for coffee in "Roy's Cafe," John sits opposite him, and if you watch closely in one of the shots, the plant behind Jim gets knocked over, yet a few shots later it's back upright standing in its container. (00:49:15)
The-ImmortalContinuity mistake: When C. Thomas Howell is looking in the mirror, in the shots taken from above the mirror, it is not that dirty. Yet, when it cuts to a close up of the mirror, it is filthy.
Answer: The windshield glass would have not broken this easily (like regular window pane glass in your home or such). I am going to guess that kind of glass hasn't been used on car winshields since at least the early 70's, if not before. The Sheriff's truck didn't appear to be that old, either.