After the Zombies are unleashed in the desert scene, we see Carlos crawling under the truck. At first his semi-automatic gun seems to be completely normal. The gun's slide and chamber are closed as if there are still bullets in the magazine or at least one bullet is chambered. But then the camera focuses on his gun and we see the gun's chamber is open - as if there are no bullets left. The gun didn't fire between shots so it's chamber should have been open in the first place.
When Alice's clone stops the lasergrid from killing the real Alice, there is a shot of the computer screen showing the disintegration of Isaacs, Dr. J. This is a reference to Jason Isaacs, who has a history of appearing in Paul W.S. Anderson movies. Anderson produced this movie, so the circle is complete.