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: 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.