During the last fight with the VC, the film changes from light to dark and back a few times.
In the cast listing David Poole(Dennis Leary) is listed as Lt. Poole. In the movie he is actually a US Army Chief Warrant Officer Three. This can be seen by the right collar bar on his uniform that has a bar with three black boxes and by the Warrant officer insignia on the left collar.
In the ending credits, the Fairchild C-123 Provider used in the elephant drop is misidentified as a Lockheed C-130 Hercules.
When BoTat and the Soldiers come under fire boating down the river, if you watch carefully, one of the mortar rounds can be seen bouncing partially out of the mortar pipe on deck the VC boat and falling back in instead of being fired out like it should.
During the negotiations to buy Bo-Tat the insignia on Doyle's beret is reversed. The insignia is of the 5th Special Forces Group and is a diagonal yellow stripe with three scarlet stripes in it going from the top left (as seen by the observer) to the lower right.) Doyle's is going top right to lower left. (NOTE: It happens that the Vietnamese flag is yellow with three horizontal red stripes.)
When the soldiers jump out of the cargo plane before the Botat, the barrels of their rifles can be seen to be bending, indicating they are props.
When the elephant is sedated and the kid pulls a knife on Danny Glover, the way the blade bends, you can tell that it's a rubber knife.
The bullet hole in Bo Tat's static line moves from the center to the outer edge.
In the beginning of the movie, the AH-1 Cobra gunship that is shown taking off for a mission is a much later variant (appears to be an AH-1F model, which came out in the late 80's) that wasn't in service in Vietnam. The giveaways are the flat canopy (most obvious), the air data sensor on the top right side of the canopy, the long 3 barrel Gatling gun in the chin turret, and the IR (infrared) jammer above the exhaust. These were all produced much later and with the TOW missile system. The earlier models used in Vietnam had rounded canopies, much shorter 6 barrel Gatling guns in the chin turret (and/or a 40mm grenade launcher) and none of the other aforementioned equipment.
As Cahill and Doyle fly over the village in the cargo plane, the blades from the camera helicopter can be seen in the top left-hand side of the screen.
Any time they are carrying their rifles, Danny Glover and the others carry properly with their finger extended off the trigger. Ray Liotta constantly has his finger on the trigger. Against all rules of gun safety.