Visible crew/equipment: In the scene when Rambo jumps on the hood of the army truck you can easily see the outlines of the cushion they put under the hood to ensure a soft landing.
Continuity mistake: The police car that Rambo rams into the stationary car with the Army truck has damage that appears and disappears, especially the rear right quarter of the car.
Continuity mistake: When Rambo is driving the army truck he rams the police car off the road. In the shot just before the police car hits the stationary car the passenger side windows are open and police are shooting at him through these windows. But in the next shot just before the explosion the cars windows are closed.
Continuity mistake: When Deputy Galt falls off the helicopter, the close-up shot reveals he still holds his rifle. A split second later (distant camera), you would expect to either see him still holding the rifle or see the rifle falling away from him. However, the rifle has disappeared.
Continuity mistake: When Rambo is being pursued through the forest and comes upon the cliff edge, the water at the bottom of the gorge changes direction of flow several times. It is first seen as flowing left to right, from Rambo's point of view. The very next shot shows the water flowing right to left. It reverses direction twice more in subsequent shots. (00:24:00)
Continuity mistake: At the onset of the film in the police station, Rambo has a few days of facial hair (also evidenced by the fact Teasle and the other officers 'wanted to shave him'). After Rambo escapes and a few hours later is about to start climbing down the side of the mountain after he hears the other officers and dogs closing in on him, look at the close-up shot of his face just as he looks back and sees that the tracking dogs are very close. Rambo now appears clean shaven. Highly unlikely he stopped to shave.
Character mistake: When the deputies chase Rambo in the forest, sometimes Teasle refers to Shingleton as Shingleton, but sometimes he calls him Singleton.
Audio problem: In the scene after Rambo holds the knife to the Sheriff's neck, we see the sheriff in the tent having his neck looked at by the doctor. The other guy, Dave, says that Rambo is now his problem. The Sheriff then replies, but the movements of his mouth do not match what he says.
Audio problem: In the helicopter scene when deputy Gault is telling the pilot "If you don't fly this thing right...." it would be impossible to hear him over the sound of the aircraft without yelling, yet Gault speaks in a normal voice.
Continuity mistake: After Rambo has dumped Robert A. from the truck, a police car takes up the chase. During the chase, the truck's headlights switch on and off between shots.
Other mistake: When Rambo turns himself in after the death of Deputy Galt, policemen fire several shots in the direction of the river. No impact of the first shots can be seen at the surface of the river when Rambo seems to be hurt by a bullet. (00:31:55)
Factual error: The military cannot be used to help execute civilian law except during emergencies. Rambo could, I guess, be considered an emergency however, once the national guard was called out, they would have had jurisdiction over the Podunk police department. When Brian Denehy told them to wait for them, and then subsequently to clean up their mess, he would not have been in charge at that point.
Continuity mistake: The rock thrown by Rambo at the helicopter doesn't actually break the windshield at all. For a very short instance (one or two frames) you can see the rock bouncing off the windshield which is still intact. The next frame is cut in (just look at the position of the pilot in the background) - the glass is now broken. Some shots later, when Galt has fallen out and the helicopter leaves, the windshield is intact again.
Factual error: In the scene in the police station, where Rambo was about to get shaved, he had a flashback of the vietcong guy slicing his chest up. The vietcong guy was noticeably speaking Cantonese instead of Vietnamese.
Continuity mistake: When Rambo is hanging from the cliff, and the officer is shooting at him from the helicopter, the cliff he is hanging from with his two arms change just prior to his jump. You see two different ledges he's hanging from, one is a totally even horizontal cliff, the 2nd is a V-shaped ledge.
Continuity mistake: During the chase, when Rambo escapes from jail, the police car jumps and the back hub-cap flies off. The next time you see the the police car all the hub caps are on. And when the police car goes through all the mud in the field, the back end of the car is all muddy, but a few seconds later it is all clean again.
Visible crew/equipment: When Rambo is jumping down to kill the boar, the camera angle is from ground up, you can see Rambo is wearing some type of white underwear over his jeans, most likely a Jock support underwear. During other scenes the brown tarp he is wearing hangs low so you can't see the white underwear.
Plot hole: The national guard leader says a dozer wouldn't be able to get to that area where the mine was blown up which supposedly was around 500 yards from where Trautman and Teasle were dropped off in the helicopter. But in the following scenes Rambo crawls through the mine which is nowhere near 500 yards long and when he comes out of the shaft he's right next to the road filled with trucks and other vehicles. So why can't a dozer get there?
Continuity mistake: The sheriff left his hat in his upside down car. He suddenly has it at the gorge.
Audio problem: After Rambo turns onto the dirt and rock road, it looks like he is shifting gears, but the sound of the engine doesn't match.
Chosen answer: Because it's a knife that someone else is holding and can use against him. It's common for people in dangerous professions to regard their weapons as tools. For example, police officers who commit suicide by handgun rarely use their service weapon, but rather a personal firearm.
Captain Defenestrator
Additionally, I believe it is made clear during that scene that Rambo had previously been tortured at some point, and one of the tools used was a knife. So the combination of being restrained and having someone approach with a knife caused him to basically have extreme PTSD and so he attempts to escape.
oldbaldyone