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: One of the cops that was with Joe Plato (how much you have seen of them will depend on which version of the movie you, but I HIGHLY recommend the fan made/edited 'Extended Mall Hours' cut on YouTube) asked them for cigarettes. You'll notice that Roger and Fran lit up cigarettes as they were taking off, that's what Roger was laughing at.