Revealing mistakes: Before Park Dong-jin "sees" his dead daughter he looks at a picture of himself and his daughter while lying on his couch. After a few seconds he re-looks at the pictures and notices his daughter is missing from the image, but you can see the picture from behind and it is obvious she is still in the picture. She only disappears from the shot after they show the picture from the front.
Continuity: During the scene where engineer Peng slashes his stomach four times, notice his strokes. He makes a semi-short stroke, then two strokes that are similar in length, and then a very long stroke on the bottom. All of these strokes are relatively straight; although, when you see the close-up of his gashes bleeding, you see that the first stroke is very short and angled down, and the bottom stroke is much shorter than the middle two, even though it should obviously be longer.
Continuity: In the ransom photo of Yoosun, she is wearing the necklace Ryu made. However, he hadn't given her the necklace when he took the photo, it was later, when he traded the necklace for her doll.
Continuity: When Ryu takes a Polaroid of Yu-sun to send to her father, she is not wearing the necklace that he made. However, in the Polaroid that her father receives, she is wearing the necklace.