(at around 1h 3 mins) Missy's pants can be heard ripping when the rapist tears them off. Yet, when we see her tied up (at around 1h 21 mins), her pants are back on her and in their original shape.
(at around 40 mins) 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.
(at around 48 mins) 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.
(at around 55 mins) When PFC "Stump" Locke is descending the cliff face, in one shot he is wearing black leather Army-style boots; in subsequent shots, he is wearing brown "desert" boots.
(at around 33 mins) When the group finds Doctor Paul Foster, the sergeant's strip-band from his helmet changes between shots from tight around his chin to hanging under his chin.
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.
Several times throughout the film, soldiers go and are sent somewhere by themselves. This is contrary to the very first, most basic instruction given to U.S. Army trainees, to never go anywhere alone.
The soldiers repeatedly address the Sergeant as "Sir." Sergeants are addressed as "Sergeant." Officers are addressed as "Sir" or "Ma'am." Saying "Yes Sir, Sergeant" is especially incorrect.
Several times they rack back a round while one is still in the chamber. All that does is waste a perfectly good round.
(at around 7 mins) 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.
(at around 57 mins) 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.
(at around 29 mins) 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.
(at around 1h 4 mins) During the confrontation with Chameleon, PFC David "Napoleon" Napoli's weapon runs out of ammunition, so he drops it. It is then picked up by Sniffer, who fires it again.
(at around 10 mins) PFC "Missy" Martinez is obviously wearing eye makeup; this is not allowed on training exercises.
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.
The soldier characters frequently "flag their buddies" - i.e., point loaded weapons at one another. U.S. Army training strenuously makes the point for soldiers not to do this.