At the beginning of the movie when being attacked in the town, a double barrel shotgun is fired 5 consecutive times w/o being reloaded. In reality it could only be fired twice w/o the ammunition being replaced.
When the crew is breaking into the department store across from the Optical store you can see another person,and older man, strolling along behind them. This man appears when Henry is breaking the lock off the store. Right between him and Mack you can see the man walking (not infected) paying no attention to the cast or crew.
The lead archer tells the group to retrieve arrows when it is safe to do so, because they are running low. In the next fighting scene, they shoot three zombies, and then run right past them without stopping to pick up the arrows.
It is stated that EMPs were used to disable all electronics and communications, which is why no one is seen driving cars in the movie and there is no power anywhere. Yet the ferry that comes to take the survivors to Catalina Island is clearly seen under motorized power.
When they are in the sport shop, looking for equipment and they get overrun, when the camera shows the zombies on the glass trying to get in the door you can see a car drive from right to left behind the zombies.
When Cassie fires the 50 cal machine gun from the shopping cart, there is no recoil. The tripod shows the spades designed to keep the weapon in place when ground mounted, but the cart should move backward or flip as soon as she starts shooting. She also has more ammunition than she found in the previous scene.
The goof items below may give away important plot points.
With all the cutting, slashing, etc. that the travelers do, no one gets any blood on themselves. Especially evident is when Henry uses a chainsaw on a zombie's neck. We see the blood spraying, but Henry stays dry.
Cassie and Henry find a .50 caliber machine gun and some belt ammunition for it. When Cassie is later shown shooting the gun, the blasts coming from the gun can be heard, but the ammunition belt never feeds through the gun.