When Peter chases after the killer out to the street, the long shot shows him from the rear, looking to his left, but in the following close-up from the front, he's looking to his right.
Just before Elsa is killed, the attacker shoves several individual wads of paper into her mouth. But as she dies, she spits it out and the pages are now wadded together in one giant ball.
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.
There are several differences between Peter Neal's apartment window's interior and exterior shots. From the interior there are concrete columns on the outside to the left and right. Also there is a painted strip along the bottom, and a low lying shelf. From the exterior there is no concrete, no paint, and Peter can be seen resting his elbow on a shelf at window height.