Main character states that lack of sunlight has left him depleted of Vitamins A and E. These are obtained through diet. Sunlight is only responsible for the production of Vitamin D. Thus, the character should only be depleted of Vitamin D.
When Lee Woo-jin recalls watching his sister fall from the bridge, there's a cutaway in which his hand is shown with half a pinkie missing, and in the next sequence, his hand is shown with all fingers intact.
There seems to be a mistranslation regarding one Chinese character. Dae Su and Mido go to several Blue Dragon restaurants to test-taste the dumplings. The Chinese characters actually translate to Green Dragon.
Oh Dae-Su states that the maxim, "Free yourself, like a gazelle from the hand of the hunter, like a bird from the snare of the fowler," is from Proverbs 6:4; these numbers are emphasized shortly afterwards. The saying is actually from Proverbs 6:5.
Despite having been imprisoned and isolated from the outside world for 15 years, Oh Dae-Su is shown to be familiar with the operation of a modern cell phone, something that would have been very rare (and completely different from the flip-top handset Dae-Su has) in South Korea when he was imprisoned in the late 80s. However, Dae-Su watched television for 15 years and it is highly likely that at some point he saw a modern cell phone being used.
When Dae-su enters the room of one of his nemises, his computer screen shows information about the stock exchange. The internet address on the screen is: C:/DocumentsandSettings/[...]/#OldBoy#/[...], thus a link to a folder on the computer, rather than an internet address
All throughout the hallway fight scene, it is obvious that the goons don't actually kick Dae-Su. Many times they kick him and his body doesn't react, while other times their kicks are even too far from his body that it appears they are kicking air, despite the fact he is motionless on the floor.
When Woo-jin Lee is searching through the Internet for the word "Evergreen", you can see that the high school website already has been visited on that computer because the link is highlighted in purple colors.