(at around 33 mins) 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.
(at around 27 mins) When 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.
(at around 2 mins) At the beginning of the film, when the radio announcer is reading Ryu's letter, she quotes him: "I'll be listening to the broadcast with my sister." But a deaf person wouldn't say that, since he can't "listen." Also, when the letter is broadcast over the radio, both Ryu and his sister are shown laughing at the words, "But you know I never do what I'm told. Ha ha ha." Again, being deaf, he wouldn't know what part of the letter is being recited at a specific time.