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. (00:25:00)
Other mistake: In the aerial shots of the semis being driven in the mall parking lot, the zombies are milling about. Notice they have left a path just clear enough for the truck. (01:02:40)
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.
Other mistake: On the initial trip to Penneys in the Mall, a zombie manages to get through the door. After Peter closes the door Roger shoots the zombie, the bullet goes through his head as the exit blood spatter suggests, but there's no entry wound.
Answer: Not necessarily, there are many instances where cars are parked inside of malls or even stadiums for promotional purposes. It is a lot easier to have gas in the car if it needs to be moved or replaced for any reason.
Invader_Gir