Continuity mistake: When the raiders first arrive at the mall which is surrounded by zombies, the zombies are standing on the curb waving their arms and standing still like a receiving line. Every other time, they attack, but not in this instance.
Continuity mistake: The Nurse Zombie and Sweater Zombie are outside of the mall, then, inside of the mall. Thus, they are among the zombies eradicated after the doors are blocked. But, they are among the zombies that invade the mall and eventually make it onto the roof.
Visible crew/equipment: After Roger says, "Lets go, baby. Number 2" and closes the truck door, you can see part of a crewmember reflected in the window.
Factual error: Towards the end, the bikers are riding out of the mall, and Peter shoots one biker out of his sidecar. 1. The biker seems unaffected despite being shot through the arm with a rifle. 2. He picks up his tommy gun (notorious for powerful recoil) and starts shooting with his torn arm. Not only is this impossible, but he doesn't even wince. 3. There is nothing in front of him to shoot, and he knew there were some behind, as he got a look around when he fell off the bike.
Plot hole: At the beginning when the SWAT team breaks through the door in the basement and all the zombies escape they don't attack the SWAT team they just run up the stairs.
Continuity mistake: When Peter and Roger are looting one of the stores, Peter brings out the two wheeled cart, which is aluminium colored first. After the shot where Steven is shown in the basement, the cart is now lime green.
Revealing mistake: When the lorries are being moved to block the entrance to the shopping centre you can see on the left in the lorry park cars driving on the freeway.
Continuity mistake: Watch when Sledge swings down from the balcony. In one quick shot, we see that there are two zombies in front of the store "Carlton's." When the shot changes to show Sledge landing, a third zombie has appeared from nowhere.
Continuity mistake: When Peter has just killed the child zombies and another with a messed face is in the doorway, we see Steven outside aiming a rifle and it is cloudy, dreary weather. Peter dives out the way and lands by a window and we see that it is now pitch black, then Roger steps in and shoots the zombie from outside and it's only just overcast.
Revealing mistake: When the SWAT team breaks the door in the basement and all the zombies escape,the SWAT team member to the far right is laughing.
Revealing mistake: When they lock the mall doors using the car, Peter is giving cover fire while the doors are being locked. Notice when the camera cuts to one of the shots from the scopes point of view he blows a zombie's head off and the blood splatters on the scope and is visible when the zombie drops.
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.
Continuity mistake: Just after the raiders manage to get into the mall, one raider uses his sledgehammer to knock a zombie wearing a black t-shirt off his feet. 5 seconds later, the same zombie can be seen walking past the same raider.
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.
Continuity mistake: At the end of the movie when Peter is running to the helicopter it changes from night to day in an instant.
Visible crew/equipment: When Roger is talking to the guy next to him at the start of the film the guy gets shot and for a brief second you can see a fake blood packet covered by make up.
Revealing mistake: When Stephen is trying to climb out the elevator after having been shot, a group of zombies attack him and the first zombie we see bites his leg. In this closeup of Stephen's leg being bitten, the tube that pumps out the blood is protruding out his trousers, obviously leading up his thigh.
Visible crew/equipment: During the ghetto holocaust at the beginning, when Willie kicks open the last door (before Peter kills him) you can see the camera's shadow on the door as it swings open.
Revealing mistake: When Peter shoots the biker out the sidecar there is already blood on the ground before he lands.
Revealing mistake: A zombie's make-up is smeared when Stephen is fighting with it on the grass at the refueling scene. Stephen is pushing the zombie's head back and the make-up on his forehead is wiped off.
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.