When Irena is alarmed by the woman in the restaurant, she makes the sign of the cross left-to-right, as a Western Catholic would. However, as a Serb, she would more likely have made it right-to-left, as Orthodox and Eastern Catholics do. And if she was Orthodox, she would join three fingers (thumb, index and middle finger) to make the sign of the cross, not use the whole hand.
Irena makes reference to the people of her village having mass, however this is only a Western Catholic term. As a Serb, she would likely be Eastern Orthodox and thus would use the term "Divine Liturgy."
When Irena does not show up at her apartment when Dr. Judd, Oliver, and Alice are waiting for her, they leave. Dr. Judd hides cane in apartment to give him an excuse to borrow Oliver's key and go back in for it, after when he leaves the door unlocked so that he can sneak back in, something hidden from Oliver and Alice. Yet after Oliver and Alice are threatened in the office, they call the apartment to warn Dr. Judd that Irena is definitely dangerous and that he should leave.
When the shepherd arrives and finds the dead sheep, there's a live sheep sitting behind him. After a brief shot of the footprints that he's examining, the film returns to a shot of the shepherd, and the sheep is gone.
The goof item below may give away important plot points.
When Oliver finds Irina's body, he kneels over her, looks up and nods his head, wordlessly confirming she is dead. His face is seen in profile as he does so, and his lips clearly never move, however the audio track has him then speak the last line of the film "She never lied to us."