Revealing mistake: When Dae-su enters the room of one of his nemesis, his computer screen shows information about the stock exchange. The Internet address on the screen is: C:/DocumentsandSettings/[.]/OldBoy/[.], which would be a link to a folder on that computer, not an Internet address.
Continuity mistake: Near the end, when Oh Dae Su is fighting with Lee Woo Jin's bodyguard Mr. Han, the latter flips Dae Su repeatedly until Dae Su very swiftly jabs Han in the ear with a pair of scissors, the blades of which are pointing upward from his fist. Han flips Dae Su one more time and that's when he feels the injury. He looks down on Dae Su and spots the scissors on his hand, but this time the blades are pointing downward from his fist.
Add timeAllister Cooper, 2011