We see 'Peter' walking down a hall, with the ceiling exit sign reading backwards, and 'Peter' looks right, then cut to inside the room, then he continues walking down the hall, walking past a fire extinguisher. Later on we see the same shot mirrored so that 'Peter' looks left and into a different room. You can tell because the fire extinguisher is now on the other side of the wall and the ceiling exit sign isn't backwards anymore.
When 'Peter', 'Anais', and 'Nurse Stark' are walking down the hallway and get surrounded by zombies as the camera cuts from looking down one part of the hallway to another you see the same background zombies threatening the heroes.
When Schreklich goes to visit his zombie wife and daughter, the door he opens to their room reverses sides from when he's outside to when he's inside.
At the end when 'Anais' and 'Peter are running through the halls being attacked by zombies, the gun that 'Anais' uses magically changes from a shotgun to a rifle.
When Dr. Gabriel Schreklich and his henchman are going downstairs and shooting the zombie, the gun switches from a shotgun to a much smaller rifle and back again several times.
When Dr. Gabriel is dissecting a frog at the start of the movie, you can see during the first close-up that the frog's left front foot has become unpinned, but in the next scene it is pinned down again, then the scene changes again to show the right front foot unpinned but once more when the scene changes back to a close-up the right front foot is pinned again.
The pump shotgun fires at least 30 times without being reloaded.
Peter is shown pulling loose a hose marked Emergency Sprinkler, then pulling loose the gas line and attaching it to the sprinkler system. First, for a building this size the sprinkler supply pipe would be 6 to 8 inches wide, not the 1 inch as shown. Second, when he switches the lines gas immediately starts venting from the sprinklers, those lines should have been full of water. Third, when he pulls loose the water supply line no water comes out of either end of the detached pipes. Fourth, considering that the sprinklers are venting all this natural gas, the shotgun blasts should have ignited it many times over.
When Peter is in a hallway shooting Zombies with his shotgun as he attempts to help his wife escape. He comes through a door, he shoots a zombie in the head, when the scene cuts to the distant shot, the hand of a set worker who is standing behind the door can be seen reaching around, holding the device that sprays the blood onto the door. After a second, the hand and device that sprayed the blood are pulled back behind the door.