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.
Continuity mistake: When Peter and Roger enter the Penney's store, Peter throws his M16 and the keys inside the store before he starts to close the door, yet Peter's rifle (Roger is holding his M16) can be seen propped against the wall outside the door twice in the following shots.
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