Other mistake: In the final scene, where Ben realises he's inside the mirror world, everything is mirrored. However, the word ECNALUBMA is visible on the ambulance. Since the word ambulance is always written mirrored on ambulances, it should have shown AMBULANCE.
Other mistake: Even though the building is said to not have power and the guards have to use flashlights, the first time Keiffer goes in the basement he is able to flick the switch and turn on the lights.
Other mistake: In the scene where the mirror cuts his hand, Ben touches the mirror with his left hand. In the very next shot he is holding the flashlight with his left, pointing on his wounded right hand. Right after that he switches hands again.
Other mistake: In the scene where the son is woken up by the "screaming woman", when the mother rushes out of her room you can see the sun in the window behind her, despite the the fact the scene takes place at night.