Factual error: When Joe Galloway arrives at LZ X-Ray, he and LTC Moore greet each other as if they're meeting for the first time. However, Galloway had been on multiple patrols with Moore before, and the two were well acquainted by the time of the Ia Drang battle. Galloway also arrived at LZ X-Ray with his own M16, which had been given to him by another officer some time before. Ia Drang was not his first time firing a weapon in combat, and he was actually fairly used to it by that time (per his own account).
Other mistake: When the black guy from their base gets shot in the head the camera slows down and you can see his forehead starts bleeding but there's no hole on his helmet. [Actually there is. If you pause it, you can see it on the (his) right side of his helmet. Although it looks as if it exploded outwards instead of inwards as a bullethole would.]
Revealing mistake: Snake Shit's helicopter has no holes in its windshield. Every other part of the helicopter is riddled with bullet holes except his perfect, spotless windshield.
Factual error: The French soldiers in the beginning of the movie have the insignia of the 1st Infantry Regiment (1er régiment d'infanterie de ligne) on their berets, the 1st Infantry Regiment never saw action in Vietnam, it was stationed in French occupied Germany between 1945-55 and then fought in Algeria. It might be a mix up with the 1er Régiment de Spahis Marocains, a cavalry unit which was deployed in Vietnam.
Revealing mistake: Towards the end of the movie, a VC soldier holds a shoulder fired rocket launcher. If you watch carefully, you can see the rocket wobbling as if it's made of rubber.