Continuity mistake: When the bikers are stealing the zombie's wallet notice Blades in the background cheering his buddy on while just standing next to a zombie. (01:48:30)
Continuity mistake: In the scene right after Francine is attacked by a Hare Krishna zombie, they are sitting, drinking and watching TV. When Roger is about to throw the bottle to Peter, he is sitting down, in the next shot after Peter gets the bottle, he is standing up and about to sit down.
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