Audio problem: Halsey tries to get the family in the station wagon's attention by honking his horn. After the near-collision with the bus, the horn honks in a different note.
Factual error: All the police officers are described on the cast as 'troopers', but those who arrest Jim at the diner and those who pull him off the bus are actually deputy sheriffs, not state troopers. They are not wearing uniform, are not driving cars with state police badges, are wearing stars instead of police shields, the place where he is taken after being arrested has 'Sheriff Station No.7' written on the outside wall, and the name of the sheriff's department can be seen when he is being photographed.
Continuity mistake: C. Thomas Howell has a mole-like thing on the left side of his chin, that appears, disappears, and fades between shots.
Continuity 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: What you see him playing with is a link on the chains around his waist - note chains, not handcuffs. We know he's a resourceful guy, and just before he escapes, we see the two guards playing cards, with the shotgun loosely on one's lap - based on what we know about the configuration of the truck, John would have easily been able to leap forward and grab the shotgun while we see Jim driving up to the truck. John then shoots the guards with the shotgun we now know he just grabbed.