Revealing mistakes: 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.
Revealing mistakes: The bars of the prison wiggle, revealing that they're not made of metal.
Continuity: 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.
Crew or equipment visible: When the last ball hits Kyun-woo's face, the cameraman's reflection is visible in the glass behind him.
Continuity: 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.