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 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.
(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.
When Romy hits the boy with her car, she closes the door when she runs to check if he's okay. When she runs to get in her car, the door is open.
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 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.
The librarian refers to the Sefer ha-Marot as a manuscript when she hands it to Casey. However, the illustrations appear to be printed, not hand drawn.
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.
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.
The contact lenses used to give the "heterochromia" effect are incredibly obvious. The blue sections cast shadows on the brown iris below.
When Romy chats with Casey via webcam, the light on the webcam on her MacBook is off, but it should to be on.
It is quite a coincidence that Casey's local library had a copy of the Book of Mirrors in their special collections, given that it is rare enough that Rabbi Sendak hadn't heard of it.
Incorrectly regarded as a goof: It has been noted that Casey would be required to wear white gloves to read the Book of Mirrors, but this is not always mandatory. Best practice is to wash one's hands but not use gloves, because the gloves make one's actions unwieldy and increase the likelihood of tearing a page.
The photo that shows the main characters uncle is a Polaroid, however Polaroids weren't invented until 1948. Her grandmother, Sofi says he died in 1944 in Auschwitz.
At the start of the film, Casey is shown as sharing her house with her father, Gordon, who is having dinner when she returns home and asks her if she wants to eat anything. However, her father never appears in any other scene set in the house, even as Casey is driven to smash every mirror there and experiences various paranormal terrors.