Although Casey is shown following and reiterating Sofi's instructions to destroy all of the mirrors in the house and then burn all remaining pieces of them, when Romy enters the home a few scenes later, the mirror over the mantle is shown with sizeable pieces remaining.
In the club scene, after entering Mark asks Casey, Romy and one other girl if he can get any a drink from the bar. Romy and the other girl orders a drink. Casey says no. As soon as Mark walks away to complete the order, Casey begins to walk through the club already holding a drink.
(at around 1 min) Casey is using her laptop on her bed, she closes it and leaves it sit, then she turns off the lamp and climbs into bed and the laptop is gone, you see a light spot in the bed that could have possibly been it, but later when she awakes from a nightmare the computer is definitely not there.
In one of the scenes where Casey is hearing noises coming from the mirror in her bathroom the camera shot is shooting right through the open bathroom door into the mirror, yet the mirror shows the reflection of a closed bathroom door.
In the opening scene, there is an overhead of a woman jogging on a bridge with snow plowed off to the side. They immediately cut to a ground-level shot, and all of the snow is melted but the bridge is quite wet.
When the professor lecturing is shown far away the word "Before" on the chalkboard behind him is completely underlined. Next time the professor is shown in his closeup, only part of the word, "Befor" is underlined. The letter "e" is clearly NOT underlined. Then afterward, he is shown in a blurry mid-shot, the word "Before" is completely underlined once again.
The librarian who shows Casey the copy of the antique "Book of Mirrors" simply requests that she "Please exercise care when handling the manuscript." In fact such manuscripts are never allowed to be handled without cotton gloves - though Casey is subsequently shown to be turning pages with her bare hands.
When Rabbi Sendak is translating the Hebrew text that Casey gave him, he appears to be turning the pages from right to left. Because Hebrew is written from right to left, he should be turning the pages from left to right.
Twins of different gender are fraternal one hundred percent of the time. Fraternal twins are obviously conceived when the mother releases two or more eggs during ovulation which are then fertilized by two of the father's sperm cells. Therefore, fraternal twins have separate amniotic sacs one hundred percent of the time and as such it would be an anatomical impossibility for the umbilical cord of one fetus to strangle the other.
When Casey is surprised by the "visitor" in her medicine cabinet and is on the floor, she leans against the door and the toilet behind her shifts indicating that it is a prop toilet, not bolted down as it should be.
In the link between the movie reel and nightclub scenes a train is shown going past in reverse. The audio is convincing, however the headlights on the train give it away. The cars under the bridge are also in reverse.