(at around 39 mins) When Ben Carson visits his sister Angela's body, he is seen holding her hand. His then moves her hand from the edge of the stainless steel table that she is lying on. You can see the condensation where her hand originally was. An extremity from a cold body wouldn't be leaving any condensation.
(at around 26 mins) Ben dusts himself down after the fire scene, and rubs his shirt down to make sure he isn't burned. If he was rubbing a clean white shirt with a bleeding hand that wasn't bandaged yet, he should get blood all over his shirt, but he doesn't.
(at around 25 mins) Near the beginning, Ben puts his left hand on the mirror. Suddenly, a large bloody gash is across his palm. When gets up, the blood and slash is gone. He catches on imaginary fire, it goes out, and his hand is still okay. When he gets home, his hand is bloody and cut again.
(at around 1h 35 mins) In the end when Ben's wife tries to provide oxygen to her son through her mouth so that he may start breathing. A doll is there instead of Ben's son when Ben's wife lips are attached with her son's lips.
(at around 51 mins) The second time Ben fires his sidearm into the mirror, he fires 6 shots and leaves 2 bullet holes. Considering how slowly the mirror originally 'healed' itself, it is unlikely that four of the holes had sealed over in the brief period between firing and when we see them.
(at around 1h 8 mins) After Ben gets Anna's file he sits down to review it. At this time his hand is bandaged from the cut. When he sits down and proceeds to review there is no bandage. In the next scene the bandage is back.
Ben returns to deal with the large mirror and fires a volley at the mirror from what looks like a Glock pistol. After the second volley, the pistol is empty. But, there are clicking sounds. A Glock, as with most semiauto pistols, wouldn't click after the last round in a magazine is fired. The slide would be locked back and the firing mechanism would be inoperable. The camera then shows a front shot of the pistol with the slide forward. You can see the breech end of the barrel in place in the ejection port. In the next scene, the slide on Ben's pistol is locked to the rear.
In the beginning of the movie, it was clearly stated that the burned-out store had no electricity. However when Ben Carson is in the basement breaking a hole in the wall, the florescent ceiling lights are on. There is also a close-up shot of an incandescent bulb flickering on and off.
(at around 1h 24 mins) Anna Esseker is sent to the monastery after being release from the mental hospital because mirrors are forbidden there. But when Ben Carson visits her, she is wearing a ring to show her commitment to the religion, the ring is clearly very reflective.
The goof items below may give away important plot points.
(at around 1h 35 mins) After the mirrors explode, Ben Carson picks up and holds the flash light as if he was holding a screwdriver. He then crossed his wrists, with the gun in the strong hand and the flashlight in the weak hand, palm up. However, the flashlight hand below the gun hand. From the front, as he enters the mirror room, he is holding the flashlight with his palm down, but the flashlight is over his strong arm. From the back, after he steps in, he is holding the flashlight below the gun. This changes two more times.
(at around 1h 40 mins) When Ben is fighting with the woman that was Anna in the basement of the hospital, he pulls a pipe from the wall, and steam pours out. For steam to come out the system would have to be pressurized - unlikely as the boiler room and hospital had been derelict for 50 years.
(at around 1h 45 mins) When Ben was "in the mirrors" the cars were still on the correct side of the street, going in the same direction. It's a one-way street, so the direction the cars are headed in doesn't matter.
(at around 1h 24 mins) Ben hands the photo of his family to Anna, a nun who is wearing a wedding ring. When a woman becomes a nun, she takes vows and considers herself married to God. Anna's wedding ring shows her commitment to her religion.
(at around 1h 40 mins) When Ben is leaving the Mayflower, firemen and other rescue crew obviously avoid him, as not to bump into him. However, it is later revealed he is not actually present at the scene.
In the final sequence at the Mayflower, evil spirits supposedly push against the mirrors and stretch them, however, the demons are clearly superimposed by CGI, since the reflections in the mirrors don't move or stretch at all.