Factual error: The movie is set in the late 60's. In the restaurant scene two guys on stage are singing "A Horse With No Name" which didn't come out until 1971.
Factual error: The language in Laos is Laotian, but Mel and all the "locals" speak Thai.
Factual error: Billy's helicopter pilot's licence is cancelled by mail. This doesn't happen. First, a pilot has his licence suspended pending a hearing. The hearing takes place, allowing the pilot a chance to defend himself. The tribunal will make its decision and the pilot is advised there and then of the result. It's like a trial, in many ways. This has always been the way, right back to the 1930s.
Factual error: When he picks Billy up with a rope from the helicopter while he is asleep, Mel Gibson flies the OH-58 (Bell Jet Ranger) helicopter single pilot from the left seat. That model, when flown single pilot, is always flown from the right since the only starter button is on the right side collective control, as well as the right side has more instrumentation than the left side.