Continuity mistake: As Hubert says, "They put the French army in an impossible situation where they couldn't win." Notice Roxanne and the people next to her, whom are bathed in sunlight, in the next shot the woman has no sunlight on her at all. (02:09:30)
Continuity mistake: Close to arriving to the French plantage, and also when leaving it, Captain Willards' helmet cover changes between a plain olive green and a camouflage.
Continuity mistake: After the 'Valkyrie' helicopter attack, there's a close shot of a soldier who shouts: 'I'm not going.' Another soldier turns back to the chopper to pull him out. Then there is a long shot of the troops at the choppers and you see the same soldier turning back to the helicopter. (00:42:15)
Chosen answer: Kurtz was protected by political connections and his military record from being relieved of command, and the Montagnard people whose village he was in were considered American allies, so they couldn't just send a force in to kill him or relieve him of command, but an individual assassin. (The US Government's "official" policy is that they do not engage in assassination, which is why "this mission does not exist, nor will it ever exist").
Captain Defenestrator