Visible crew/equipment: During the motorcycle chase Rambo jumps over the train tracks, if you look close you can easily see the ramp. (00:18:00)
Visible crew/equipment: In the shot of the stolen army truck crashing through the police road block, you can see a camera mounted inside the truck. Look in the small window on the rear wall of the cab. (Apparently both this shot and the shots from inside the cab were done in the same take.) (01:13:10)
Visible crew/equipment: At the beginning of the movie, Sheriff Teasle escorts Rambo to the outskirts of town. As the sheriff drives off, the boom mike is visible in the reflection of the car's right rear passenger window. (00:07:30)
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.
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