On the telephone, the killer tells Peter Neal that "you wrote those words, page 46," but in fact the words quoted would have had to be on an odd-numbered page of the book TENEBRAE, given the placement of the text we see in the opening sequence.
The goof item below may give away important plot points.
When Peter Neal is confronted by Detective Germani after killing the female detective (Altieri), her "dead" body is lying on the floor in front of him and her eyes blink. We then cut to Germani who is holding his gun on Peter. When we cut back, the corpse's head has changed position. Then again, after a couple more cuts back and forth, her head has reverted to it's original placement.