Continuity mistake: When Rambo crashes through the road block in the army truck it's still daylight outside but when he's approaching the gas station it's nighttime. How could so much time pass when he was within minutes of town when he went through the road block?
Continuity mistake: In the scene when the gas station blows up, watch the cars parked in front of the "Kool Lights" advertisement. These two cars explode (for no obvious reason, by the way) after the police officers have gotten out of their cars. They run away from the explosion and leave their police cars behind. Next shot, the same two cars in front of the ad explode again.
Continuity mistake: When Sylvester Stallone is in the stolen army truck and crashes through the police road block, the truck becomes airborne. Upon landing, the gas tank is clearly separating from the truck, but in the next shot, the truck and gas tank are together.
Continuity mistake: When Rambo approaches the police road block, the truck's windshield is penetrated by bullets. When the truck crashes through the barricades, the bullet holes are gone. They reappear on the next shot, but at the wrong places, then disappear again.
Continuity mistake: Following his encounter with Rambo in the woods Teasle has a long bloody cut on his lower face. When he returns to his office the cut is completely healed and gone. But when he sits down in the bar with Trautman it is back. After this scene it vanishes for good so I presume the bar scene was originally intended to be inserted before Teasle returned to the office.
Continuity mistake: When Sherrif Teasle is chasing Rambo on the stolen motorcycle, Teasle drives his police cruiser through a small ravine. The car hits ground hard and the lens to the driver-side front turn-signal can be seen popping out of the fender. The lens is mysteriously back in place in all subsequent shots.
Continuity mistake: When Rambo blows up "The Outpost", Rambo is in an alley and runs for cover behind a mail box. A couple of shots later Rambo does this all over again, but this time the shot is reversed.
Continuity mistake: In the opening scene when Rambo is looking for Delmar Berry, a woman is hanging washing on a clothes line. In one shot she is hanging an orange checkered towel. A moment later in the next shot she is hanging the same towel in the same place again.
Continuity mistake: When Teasle talks to Trautman for the first time, Teasle is wearing a winter coat over his police uniform. At one point the brown lapel of his police jacket is sticking out past the collar of the winter coat, but in the next shot it's tucked underneath, and he didn't have time to fix it.
Continuity mistake: During the escape from prison, Rambo grabs a motorcycle and is being chased by a sheriff's car. During the chase, the rear view mirror in the sheriff's car disappears and reappears throughout the scene.
Continuity mistake: When Orval (the dog man) gets shot, Sheriff Teasle and his deputies get down seeking cover. Teasle's hat falls down to his left-hand side. Next shot (close-up), his hat falls down again, this time to the right-hand side.
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.
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.
Continuity mistake: When Rambo has Teasle up against the tree with the knife, it is under Teasle's throat, but in the next shot the knife is to Teasle's right without Rambo moving it.
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.
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.
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