Factual error: The date on the top of Chef's letter from Eva says, "Sept" with the year conveniently whited out. If the letter was written in September of 1969 she couldn't have known about Manson who wasn't arrested until December 3, 1969. This also dates the movie as taking place sometime in 1970. (01:50:50)
Factual error: Captain Willard recalls back to lunch at General Corman's and we hear Harrison Ford's voice say as he points to a map, "Now he's crossed into Cambodia". The town he's pointing at (Ban Me Thout) and the one above it are in the Central Highlands in the middle of Vietnam and not crossover points anywhere near Cambodia. (01:18:20)
Factual error: There are two factual errors in the helicopter attack scene - for the time frame of the war. First, the helicopters carrying troops would not have been armed with rockets or miniguns, these helicopters were called "slicks" and would only had door mounted machine guns. The helicopters armed with the rockets and miniguns were "gunships" and would not have been carrying troops. Second, a Huey is shown carrying in the Navy river patrol boat (PBR) slung beneath it. At over 16,000 lbs., a PBR weighs over four times more than a Huey of this era could have lifted. (Regardless of the REASON why these mistakes occur in the film, they are STILL mistakes.)
Factual error: In his final speech to Willard, Colonel Kurtz talks of the Vietcong's "genius" for chopping off the arms of children inoculated against polio. While polio was technically available in both an oral form and an injectable form, any soldier administering the vaccine in SE Asia during the time would most certainly be administering the oral vaccine, and not giving a shot in the arm.
Factual error: In the scenes where members of the boat crew are wearing berets, they are worn incorrectly. Both Clean and Chief Phillips have their berets with the insignia over the right eye with the rest of the beret slanting over the left ear. That is the manner in which the French armed forces wear berets. The US and British forces wear them with the insignia over the LEFT eye with the rest of the beret slanting over the RIGHT ear. Ask any Ranger or British soldier.
Factual error: Captain Willard narrates to himself, "I am beyond their timid lying, morality, and so I am beyond caring" as we see a downed helicopter burning in a tree. The main rotors are detached and would not be sitting on top of the helicopter had it actually crashed and not just been placed there. In an actual helicopter crash the rotors fly off. (01:22:10)