The deserter is carrying a Daewoo K2 assault rifle when he takes Kyun-woo and the girl hostage, but at the merry-go-round he suddenly has a Colt M4. While the girl is talking to him, the M4 is briefly replaced by a Colt M16.
The girl's hair is tucked behind her right ear during and immediately after her piano performance, but when Kyun-woo gives her the flowers her hair is hanging loose.
When the girl is waiting to board the train at the end of movie, the camera initially shows only her feet and legs with a young woman in a white shirt and blue jeans in the background. When the camera cuts to a view of the girl's face, the woman in blue jeans has been replaced by a woman in a long skirt.
When Kyun-woo is browsing the Internet and checking his e-mail, the address bar shows that he's looking at local files on his hard-disk, not actual websites.
The bars of the prison wiggle, revealing that they're not made of metal.
When the last ball hits Kyun-woo's face, the cameraman's reflection is visible in the glass behind him.
In the metro (foot) scene, after Kyun-woo hits The Girl and during The Girl hits Kyun-woo, the windscreen of the boom microphone is clearly visible.