Revealing mistake: When Stephen first approaches Fran, who's manning a television camera, he says "meet me on the roof at 9 o'clock. We're getting out in the chopper." She replies "What? David we can't", referring to the character, Steven, by the actor's real name (David Emge). (00:05:10)
Revealing mistake: When Peter shoots the door the first round goes through, but the second shot just shows a flash of light behind the piece of paper on the door and never goes through. (00:22:40)
Revealing mistake: After Peter and Roger pull away in semi-trucks, traffic can be seen behind them on the road. (00:59:16)
Revealing mistake: The female zombie grabs onto the back of the car and is dragged through the mall. Stephen shoots the zombie in the eye and if you look closely you can easily see this is no longer the woman from before, but actually a man who doesn't resemble her at all.
Revealing mistake: When the bikers are grabbing their weapons, we see a grenade. You can tell that it is fake because the bottom is bored out.
Revealing mistake: When Peter (and Stephen) are locking all the doors, Roger gets his rifle from the back of the car and shoots a zombie. Even though the bullet goes into the side (two-inches away from the middle of the zombie's head) a small amount of blood shoots out the middle, and something at the edge breaks off/moves, revealing that it's a tomato sauce sachet stuck to the actor's head.
Revealing mistake: In rural airport scene, when Peter reloads his weapon, he pulls a small, narrow magazine out of the bottom of the m16 mag, thus indicating that the weapon he's using is some sort of lightweight 22LR copy of m16 assault rifle, not an actual m16.
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.
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.
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.
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.