Factual error: When Rambo is going through the Soviet minefield, he probes a couple of times very hard with his knife, and then he lunges forward. Now, jabbing hard into the ground where you think there might be an explosive device is as bad an idea as it sounds. Also, Rambo doesn't probe to the sides where his elbows and knees are going to support his weight. Also, he lunges forward farther than he had probed. And finally, the mine he does find is very big for an antipersonnel mine. It looks big enough to be an antitank mine.
Suggested correction: Whilst generally speaking, you are correct, Rambo jabs in at an angle for the mines which is a technique still used today by some special forces. Many works of fiction written by real-life special forces members will state this method. If the producers had chosen to show Rambo clearing the minefield properly, the scene would have taken much longer than it does. The mine at the end, may well be an anti-personnel mine however, this does not make it a mistake, it would still do the job required.
