When Jim is watching TV in the barn office the rifle is clearly visible lying on the sofa behind him. In the next scene he finds the rifle in the gun cabinet for the first time.
When the Zombie is shot trying to get upstairs in the warehouse towards the end of the film, the blood-splatter pattern on the door is at roughly the same height as the sign saying 'Stairs'. In the proceeding shots, it is much lower.
When the Zombie is shot trying to get upstairs in the warehouse towards the end of the film, it slumps in to a sitting position with its back against the door. In the proceeding shots, it is laying on its front, through the doorway. There is also no sign of a bullet wound on the back of the head.
In the beginning when Jim is inside the barn and he is yelling about having a gun there are no foot prints on the ground, after seeing the cat and turning around do the foot prints appear. They are also backwards leading towards him.
Every time they show the rifle, the scope has lens covers still in place. There is nothing to see, It covers both the front and back of the scope.
Scott says their warehouse has no windows so they can keep the lights on whenever they want, but when they phoned the barn, they flashed their lights through windows so Jim could see them.