When Matt starts confessing to Shane after getting hit in the head with his own helmet, his face is clean minus a few scrapes by his left eye but it in the next shot he's sweating and has blood coming out of his nose and mouth.
When Shale meets the crew at the pistol range, Hollen (William Forsythe) is wearing a shoulder holster that has two thin straps and two wide straps; he is wearing the holster upside down, with the wide straps under his arms. This holster is designed to be worn with the wider straps over the top of the shoulders to spread out the weight and pressure.
When the mercs are at the firing range, there are several shots of Shale with people behind him "shooting" a Glock. If you take notice, you can see they are simply imitating the recoil by slowly flipping the gun back.
At the gun range, the woman in the white shirt shooting a pistol behind Shale is not pulling the trigger, the gun's slide is not moving and there are no cartridges being ejected. She is only moving the gun slightly up and down to make it appear as recoil.
The goof item below may give away important plot points.
Crew or equipment visible
In the school shootout scene at the end, Joey fires a rocket at the two mercs who are hiding behind the lockers. Right before the rocket hits you can see the 'yank rigs' and cabling that send the stuntmen flying when the rocket explodes.