(at around 18 mins) Just before the scene where Dae-su Oh meets the suicidal man at the top of a building, there is a part when he finds out he wears an expensive suit and a watch. When he grabs the man however, the watch is gone.
(at around 1h 50 mins) 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.
(at around 1h 30 mins) The last scene at Woo-jin's penthouse, while he is getting dressed, his shirt cuffs already have cufflinks in them; in the next shot, they are missing, and he is putting them on.
(at around 35 mins) There seems to be a mistranslation regarding one Chinese character. Dae-su and Mi-do go to several Blue Dragon restaurants to test-taste the dumplings. The Chinese characters actually translate to Green Dragon.
(at around 1h 26 mins) 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.
(at around 34 mins) The common surname "Ljungberg" would never been used in conjunction with the honorific "Von" and "Von" is very unusual in Sweden.
(at around 34 mins) The address for "Eva Von Ljungberg" is incorrect both in format and factually. The street address while correct is not residential building, it is a hotel. Post box is almost never used in conjunction with a street address. The zip code is wrong, it should start with 12 not 10; zip codes starting with 10 are used for large companies and not coupled to a physical address as shown in the note.
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.
(at around 1h 7 mins) When Mi-do 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.
(at around 40 mins) 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#/[...], thus a link to a folder on the computer, rather than an internet address.
(at around 43 mins) 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.
(at around 1h 50 mins) When Woo-jin Lee was pulling Lee Soo-ah's hand on the bridge, there was an angle that showed Yoon Jin-seo's dummy stunt double.
(at around 28 mins) When Oh Dae-su passes out after eating the octopus, you can see camera equipment reflecting in the silver platter dish as the scene zooms out.
(at around 29 mins) 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.