Current U.S. Army guidelines mandate that men's hair be not longer than 1/4 in. on the back and sides on the head, and not longer than 2 in. on top. PFC David "Napolean" Napoli's hair is obviously longer than this.
When Napoleon, the "Eagle Scout," ties the knots in the rope, a closeup reveals him clearly tying a double figure eight knot. When the camera moves to the medium shot, he has magically tied a double bowline.
In one shot of PFC Napoli trying to reach Sgt. Millstone on the radio, his shoulder patches are on the wrong side, and the flag patch has an incorrect orientation, with the union on the left. This suggests that the image must have been reflected, accidentally or otherwise.
Missy gets caught by one mutant while she's having a leak. Yet when she is lifted by the mutant and carried away her pants are up and not down as they should have been if she didn't wanted to soil them. This is even clearer to be seen when she is then shortly after laid down in the room inside the mine. We then there see that her pants are not even up, but that even her belt is buckled/tied. It would be very doubtful the mutant picked his time rearranging her pants while dragging her along.
During the scene where PFC Missy and PFC Amber were sitting on a rock and pretending that they were chatting in order to lure the wild mutants into a trap is shot in the late afternoon. Because of the light orange-red background we can clearly see that it is at near the setting of the sun. However, from the next and following scenes to the end of the film, in the dark mines we can see clearly that the sun was shining brightly, which means it is at noontide.
Every U.S. soldier is trained, often through repeated "corrective action", never to let his or her weapon out of his or her sight. The characters do this frequently, even before they encounter the mutants.
The rifle scopes are mounted to the carrying handles without a spacer adapter to level them. As a result, all of the scopes are canted steeply downward, following the slope of the carrying handle. If you looked through a scope mounted this way, you would only see a spot on the top of the rifle barrel, midway along its length.
Members of the team frequently refer to themselves as members of the "U.S. National Guard." There is no such thing. Each state independently maintains its own National Guard, and it would be addressed as the "New Mexico National Guard."
When the Colonel asked the soldiers about their CO, the soldiers replied that he was dead, referring to the sergeant. CO is the Commanding Officer of a company, who normally holds the rank of Captain. An E-5 Sergeant cannot command a company.
In the mine, "Napoleon's" M4 carbine is shown firing in continuous full auto. While the M4A1 does have this function, its issue is limited to select units (e.g., Rangers, Special Forces, Asymmetrical Warfare Group, etc.). The standard issue M4 is limited to a three round burst, just as the M16A2 and A4 are.
The soldiers are wearing the three color Desert Combat Uniform (DCU). By the time of filming, this uniform had been replaced with the digital Army Combat Uniform, and the wear-out date of the Desert Combat Uniform (as well as the woodland pattern Battle Dress Uniform) had passed.
In the training scenario where we first meet the National Guardsmen, a force-on-force exercise is conducted using what appears to be live ammunition, live hand grenades, and live ordnance. For reasons which should be so blatantly obvious as to not require explanation, this is not done in practice. Furthermore, a woman emerges from a building in which two live hand grenades had just been detonated - again, not done in practice. As Hollywood-type special effects of the sort used to simulate bullet strikes are not used in training scenarios by the military, due to cost and lack of usefulness, the use of live ammo is the only conclusion which can be made here. Were they using blank ammunition, blank adapters (yellow for the M4 carbine) would have been affixed to the end of the rifle barrels.
The PASGT Kevlar helmets worn by the National Guardsmen have chinstraps which extend both in front of and behind the ears. This type of chin strap was only used by the Airborne and other active jump status units with that helmet type - non-Airborne troops had a simpler chin strap which only extended in front of the ears. Furthermore, those chin straps were olive drab in color, not black.
After the training exercise, and After-Action Review is held. During the AAR, magazines are still clearly seen in the rifles. The FIRST thing done at the conclusion of a live fire exercise it to lock and clear all weapons. Ammunition is then turned in before troops exit the range - the military keeps extremely tight controls on ammunition in a training environment, and does not give soldiers free reign to simply run about while carrying live ammo. This error is further continued by the fact that they possess live ammo when they arrive at the scientists' camp.
In the scene inside of the mine, when PFC Amber takes a round out of her rifle magazine to "save one for herself", the primer on the round has already been fired, as can be seen by the dimple in the primer.
After Sarge has been shot, the trainees are freaking out because the mutants have stolen their climbing ropes and they can't get back down. Why didn't they go back down the way that Amber and Napoleon had just come up without any climbing gear?
When they began the chase underground, Amber takes a bullet out of her rifle's magazine to save it for herself. The firing pin mark can be seen on the primer of the round, indicating that it's a spent cartridge with a dummy bullet inserted.
The goof items below may give away important plot points.
After the sergeant is shot in the chest, PFC "Missy" Martinez gives chest compressions to revive him, without removing his body armor. U.S. Army Combat Lifesaver training teaches not to give chest compressions when there is a massive chest wound; additionally, the body armor would render the chest compressions utterly ineffective.
The saved M4 round that PFC Amber Johnson pulls out of her pocket near the end of the movie is actually a blank round. You can tell by the way the tip doesn't come to a point. It is crimped at the end with no bullet in it.