Continuity mistake: Near the end, Steven is in an elevator, and the zombie opens the doors and starts biting him. When he gets them all out of the elevator, blood comes out of his mouth and down the right side of his face. But later the elevator doors open and Steven is a zombie and the blood is on the left side of his face.
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.
Other mistake: At the small airport after the heroes flee Philly, Peter walks into the office and is startled by a sound beyond the door. Thinking it is a zombie, he begins to fill the door with holes, about five of them. On the last shot, the door merely chips and does not leave a bullet hole. This can only be seen in the DVD version. (00:25:00)
Visible crew/equipment: In the rural airport scene a zombie is killed as Roger is filling the helicopter with fuel. As the zombie falls off the boxes, the tubing that pumped the blood is coming out of his pantleg. (From the Divimax widescreen DVD) (00:24:00)
Deliberate mistake: When the zombie (Tom Savini) is hit by a truck and spits a bunch of blood on the windshield, when he gets hit, the truck isn't moving at all. He runs into the car and spits blood to simulate being hit. It's hard to catch, but it's more obvious in slow motion. (Mentioned on DVD commentary.)
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)
Continuity mistake: Early in the movie, Peter enters the basement and has to exterminate zombies in a cage. He fires seven bullets from a six-chambered revolver without reloading. (00:15:05)
Continuity mistake: After the doors are closed, all the remaining zombies are exterminated. In several shots we see prone bodies with puddles of blood around the head. Later, when they are cleaning up the bodies, there are no blood pools around any of them. (01:20:00 - 01:23:30)
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 zombie that attacks the helicopter at the gas station loses the top of its head to the spinning rotor, causing gouts of blood to flow down its front. In the shot as it falls over, the blood has miraculously stopped flowing and is now a dry stain. (00:25:20)
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.
Revealing mistake: After Peter and Roger pull away in semi-trucks, traffic can be seen behind them on the road. (00:59:16)
Visible crew/equipment: When Peter and Stephen have finished locking the whole row of "pretty indestructible" mall doors, Peter sets the security alarm. Soon after, the team drives off. A few zombies are shot on the go. As a wounded Roger skids the car to a stop in front of some exit doors, it passes an undecorated space with glass doors right before "John's Shoe Repair." There's a cameraman standing in the back of the space, with another two crew-members squatting down next to him against the wall. (01:25:35)
Continuity mistake: In the scene where Roger gets the bag and drop it on the floor whilst trying to get back to the truck, he gets bitten on his left arm. But in the next shot the bite mark is gone and then the bitten sleeve is on the right side. (01:08:05)
Visible crew/equipment: In the scene where Roger and Peter are getting the trucks to block the mall entrances, in one shot when they are both in the same truck you can see, in the side window, a crew member is driving a white van. After Roger gets out of the truck the van is gone. (00:55:00)
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.
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)
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.
Continuity mistake: When they stop to refuel the helicopter at the abandoned gas station, a zombie approaches from under an airplane. In one shot, it is holding onto the wing support next to it - in the next shot, the zombie is standing in front of the support with its hand at its side. (00:22:25)
Continuity mistake: After the four arm themselves, they go to raid Penney's. As Peter and Roger roll up with the wheelbarrow, Francine shoulders her rifle. There is a cut and she shoulders it again. (01:13:35)
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.