Revealing mistake: There are no keys in the Jeep when we see the breakdown happen. You can see the steering wheel and where the keys should be. How is this car even moving? (00:08:05 - 00:08:35)
Revealing mistake: When Kurt is being chased after in the truck at the end, he pulls out onto the main highway and the truck skids. There are several tire marks where they practised this skid before. Also, at the very end when the truck is being pushed by the semi, look ahead of the front truck tire, more previous tire marks. (01:21:20)
Revealing mistake: During the ending sequence, just as J.T. Walsh is pushing the truck on the bridge, there is a shot looking at the front of the semi and you can see that it is not J.T. Walsh driving. The stunt man is noticeably heavier than J.T. is, and his hair is messier.
Revealing mistake: Near the end, when Kurt Russell and his wife are in a beige utility truck which is being rammed through a bridge railing by the semi-tractor, there are at least two close up shots in which miniature models were very obvious.
Revealing mistake: When Jeff and Amy are being chased by Red near the end and Red's truck begins to jackknife, you can see how they made the trailer slide in that direction. It seems that they constructed or used a trailer that had a set of wheels that would turn. They were turned in the direction of the slide.
Revealing mistake: When Kurt is floating down the river after escaping the vehicle, his shirt looks much bigger than normal, indicating he is wearing a flotation device.
Revealing mistake: When Kurt Russell is climbing the side of truck you can see a fence revealing that they are parked, even though wind is blowing to make it look like they are moving.
Revealing mistake: During the chase on the dirt road, Jeff pulls the Jeep up to the edge of a cliff. Look at the ground ahead of where he ends up. There are clear tire tracks, and the Jeep follows them to a T. This was obviously where they practiced the scene before.
Answer: There's only one major road in and out of the town. They knew where the Sheriff was coming from (Red called in an accident out of town to get the Sheriff out of their hair). If they start driving along the road the Sheriff took to get back to town eventually they'll find him.
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