Continuity mistake: Visible on the theatrical release. After David has overtaken the truck, you can see from the interior shot that David has his seat belt on. A few seconds later in the external shot David is no longer wearing his seat belt. (00:08:10)
Continuity mistake: Near the start of the film, David stops at the gas station. We see him get out of the car and he asks the attendant if he has got change for the telephone. We can see David take out his wallet and pull out a note with the other hand, then puts his wallet away. Then the camera angle changes and his wallet is back in his hand again and he puts it away again. Also you never see the attendant take the note out of Davids hand. It just disappears. (00:10:45)
Continuity mistake: When David drives away from the petrol station near the start of the film, you will see the truck catch up with him in the next scene. Take a look at Davids parcel shelf in his car when you see the truck fly round the bend, and you will notice a white baseball cap and a pair of sun glasses on it. They were not there before and also not there for the rest of the film. (00:14:20)
Visible crew/equipment: Visible camera on the right side (near the tail lamp) of Mann's Plymouth (before the Chuck's Cafe scenes). (00:22:20)
Continuity mistake: When David crashes his car into the fence at Chuck's cafe, it shows that it was the passenger side that crashed into the fence. The driver side is undamaged. However when David exits his car to help the bus driver, there is significant damage to the driver's side of his car. (00:26:40 - 00:44:25)
Continuity mistake: When David has the accident and crashes his car against the fences, he steps out of it and closes the door, but when he comes back to go on his own way, the car door is inexplicably open. (00:26:49 - 00:42:52)
Other mistake: While in Chuck's Cafe, a creepy shadow of a woman enters the camera view and it seems that she is playing the jukebox. No music ever plays in the place. (00:32:00)
Visible crew/equipment: When Mann's bumper gets caught under the school bus and he's trying to free it, a large lighting panel is reflected in his car door every time he opens it. (00:47:00)
Continuity mistake: After the truck tried to make David's car hit by the train dabbing it from behind, the car has a dent on the right rear light, but in a following scene, the dent has vanished. (00:53:09 - 00:54:30)
Visible crew/equipment: During the scene in the gas station with snakes, as Dennis Weaver is wiping the tarantula from his leg, look at the shadow of the old woman behind him. There appears to be a moving crew member shadow next to hers. It cannot be a tank as she wasn't standing next to one either in the shot before or after. (00:55:50)
Revealing mistake: At the reptile farm, the phone booth Mann uses to try and call for help has no phone lines attached to it, above or below ground (you can see that there are no underground wires when the truck mows the booth down). It's definitely not a wireless/radio telephone, either. There should be lines running to it, but there are none. (00:57:00)
Visible crew/equipment: Just after the truck has run down the phone booth at the Reptile lady's place, it starts smashing into all the glass reptile cages nearby. If you look closely at the reptile cage which David falls over, you can see the film crew in the reflection. (00:58:05)
Continuity mistake: When David returns on his way after sleeping, he first wears his glasses in a shot, but when he suddenly brakes after seeing the truck stopped on the side of the highway, his glasses are gone. (01:03:07)
Visible crew/equipment: When Mann's car spins out on the gravel and comes to rest facing the truck, you can briefly glimpse the cameraman (purported to be Steven Spielberg himself) crouching in the back seat. (01:04:25)
Continuity mistake: Mann's '71 Plymouth Valiant has the ignition slot in the steering column. But in one insert shot of his speedometer as he races the truck at 100 mph, an older model's dashboard ignition slot is visible - with no key in it. (01:10:00)
Continuity mistake: At the end of the film, David Man comes to a dead end, so he spins the car around and waits for the truck to appear. When the truck appears from over the hill, take a look at the very next shot of the truck from the front, and you will notice that the large exhaust on the side of the truck has moved from left to right and back to left again. (01:21:00)
Visible crew/equipment: When Dennis Weaver is in the cafe a shadow of a cameraman is cast across the table he is sitting at. The shot then changes to a wider angle to reveal that there isn't anyone near him who could have cast that shadow.
Continuity mistake: On the truck cab's rear window, there is some sort of air conditioning unit installed. In some shots, such as the gas station scene where David Mann is on the telephone, or the railroad crossing scene, the air conditioning unit has a support structure to hold it in place. In the rest of the film however, there is not any support structure to be seen.
Visible crew/equipment: When David's car gets stuck on the bus bumper, the next shot is of the bus' black bumper and the side of the car. At the top of the black bumper there are two people seen in the reflection. The person on the left turns and takes a step to the right.
Revealing mistake: Just before the truck slides off the cliff edge, you can see that the truck's cab door is open where the stunt driver jumped out.
Answer: At that point in the film, the protagonist David Mann is ready to confront the truck driver. When he sees the old Peterbilt truck outside, David mistakenly assumes the truck driver has already entered the diner, so he confronts a likely suspect that he sees at the counter (but he has misidentified the man). The misidentified man takes offense and punches David out. By the time he recovers his senses, David sees the old Peterbilt truck leaving the parking lot. Which means the actual homicidal truck driver never entered the diner in the first place and was waiting outside the whole time. If David had first gone outside to the Peterbilt, there was a good chance the waiting homicidal truck driver would have killed him right there, and the story would have abruptly ended. So, David's misidentification of the truck driver allowed the film to move ahead into its next act.
Charles Austin Miller
Yes, I get why the filmmakers did that, but I still think it is a plot hole. If the Dennis Weaver character was afraid of getting killed by the truck driver, I doubt he would have confronted him in the cafe.