The doctor that Sofia is talking about as she recounts her time in Auschwitz is the infamous Dr. Josef Mengele. He was notorious for selecting those that came off the cattle carts for those who lived or were sent to the gas chambers. He was particularly interested in experimenting with people, whether it be freezing people in sub-zero water or changing the color of the iris of eyes. He also had a deep interest in twins and would do organ switching, blood transfusions or sewing twins together.
When Rabbi Sendak opens the Hebrew book in his office, he thumbs through it in the wrong direction. Hebrew is written in the opposite direction to English; a real rabbi would have opened the book and read it from right to left.
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.
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.
The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. Spoiler tags have been used sparingly in order to make the page more readable.
If the demon can only possess Casey, how did it possess the little boy Matty?
The demon can possess anyone, but it targets Casey. As seen throughout the film, numerous people are possessed at different times. It uses whomever it makes contact with as a "vessel", but it wants to be BORN through Casey. It seems that it can force its way to our world through mirrors, which is how it manages to possess people. In order to become a physical being, however, it needs to possess an unborn baby and be birthed naturally.
Is "The Unborn" based on a book?
No. The Unborn was filmed from an original screenplay by American screenwriter David S. Goyer. Goyer also directed the movie.