When the guys leave the gun store, they stand around and talk for a minute. During this scene, Ferdy alternates between holding his revolver with two hands and holding it with one hand and a propane tank in the other.
The gasoline can that Allison shoves towards the robot spreads a pool of burning gasoline, but nothing caused the can to rupture. To be effective, gasoline bombs need to be made from something breakable.
When Alison is getting attacked in the pet store by one of the killbots, several tarantulas and even a snake crawl on her as she hides under a shelf, all clearly live animals. Once the killbot leaves and she emerges from her hiding place, it is clear the spiders placed on her are just plastic toys as they fall off with no movement of their own and are much smaller than the live tarantulas.
The goof items below may give away important plot points.
When Leslie gets concerned that Mike hasn't returned with her cigarettes, she goes into the mall looking for him. Right around the time she finds him you can see a side-shot of Mike's dead body where he is clearly moving his toes even though he's suppose to be dead.
During the scene where they first show Alison hanging from the railing it's suppose to appear that she's reaching for a part of the railing. The shot is not edited enough because you can see that she's not actually reaching but just flexing her hand.
After Rick is killed by one of the killbots, electrocuting himself in the process, there is a shot of him lying down on the ground, gun in hand, next to the malfunctioning killbot. However, it is clearly just a mannequin with a gun placed on top of its hand.