Other mistake: At the small airport after the heroes flee Philly, Peter walks into the office and is startled by a sound beyond the door. Thinking it is a zombie, he begins to fill the door with holes, about five of them. On the last shot, the door merely chips and does not leave a bullet hole. This can only be seen in the DVD version.
Other mistake: During the truck scene, when zombies attack Roger the first time, as Peter shoots the zombie that Roger is fighting, he supports his M16 from its carrying handle, thus blocking the rear sight (which is located in said handle). He does this in several scenes throughout the movie, but in the truck scene accuracy is crucial.