Factual error: When John is going back to the pirates boat to set it on fire you can hear his boat's engine running. However in the closeup the pulley for the water pump is not turning. This means the engine isn't running.
Factual error: An unexploded British 'Tallboy' bomb from World War II is found in the jungles of Burma. First, no Tallboy bomb was ever dropped in Burma. Further, the purpose of the Tallboy is to quake the earth to shake concrete buildings down, so dropping one in Burma's jungle would be pointless. Lastly, if a Tallboy had indeed been dropped, it falls at great speed and is hardened to penetrate concrete structures, embed itself below them, then detonate. It would have been underground, not tangled in branches above ground as if made of balsa wood, for Rambo to easily find.
Factual error: During the night, School Boy shoots a Barrett M82. Instead of hearing a huge blast (as in the morning scene), the shot is silent.
Suggested correction: They make suppressors for the M82.