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.
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)
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.)
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)
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)Phoenix