Patzer

When Julian is getting ready to use the butt of the shotgun to break into the room where the keys are, just a split second before he hits it the window shatters as if a small explosive is used as to make it appear he hit the window.

Throughout the original "The Return of the Living Dead" trilogy, shooting zombies in the head did not kill them, which was a key plot point. Yet in this film, it seems to work. However, the name of the gas changes from 2-4-5 Trioxin, to Trioxin 5, which may indicate that these zombies are different. This doesn't explain out severed limbs can be animated by Trioxin 5 though, and is most likely just an example of poor film making, or trying to adhere to the currently accepted norm in zombie films.

(at around 16 mins) When Garrison pumps the gas into the container to re-animate the severed arm, you can see the gas leaking out into the room. That should, but doesn't, turn him into a zombie.

(at around 43 mins) When they are in the air ducts asking for directions over the radio, we see the map a girl is looking at and the compass is wrong. Instead of being North at the top, East on the right, South at the bottom and West on the left, it shows North on the top, West on the right, South on the bottom and East on the left.

When Julian is about to smash the window in the parking garage, just before the butt of his rifle hits the window it shatters as if already triggered early.

In the scene where they kill the first two homeless zombies, The second one gets shot in the head twice but only one gun shot is heard.

(at around 16 mins) When the severed arm is being re-animated in the chamber, the rubber mat on the bottom or the container can be seen lifting with the actors arm as he lifts it.