When Detective Seo Tae-Yoon's gun is tossed away from his hands by Park Hyeon-gyu, the gun is seen landing to the left of the train tracks from the camera's point of view. Moments later, when the detective returns to pick it up, the gun is seen lying on the train tracks itself.
In the letter received from the FBI written in English, there are four spelling mistakes - 'examnation' instead of 'examination', 'insrumental' instead of 'instrumental', 'dateed' instead of 'dated' and 'Octorber' instead of 'October'.
When Detective Park shows murder suspect Kwang-ho a picture of the footprint in the dirt, Park is also holding a right foot sneaker that matches the print in the picture. However, the footprint in the picture would have been made by a left foot sneaker. The print and the correct shoe making it would have been reverse images of each other.