Revealing mistake: In one of the flashback scenes at the "bunker", a number of the shots fired pierce the sheet metal roof. When they show these holes, you can see fairly bright light coming through. This scene was supposed to be in the jungle during the night. There shouldn't be light streaming in; moonlight is not that bright. It was obviously artificial light used to enhance the scene.
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.
Revealing mistake: Near the end (DVD), when Sadako comes out of the TV, you can see where her finger extensions have been placed (you can see the join between her real finger and the fake one). (01:22:50)
Suggested correction: She's not wearing finger extensions. There are small prosthetics to make it look like her fingernails are missing, but her fingers are normal length.
Revealing mistake: When Damian knocks Lee Remick over the balcony with his tricycle, five big goldfish in a bowl get knocked over, too. But the fish don't move; they don't flap around like fish do out of water. They are obviously fake fish.
Revealing mistake: You can see Jackie's knuckles are already bloody BEFORE she starts pounding on the closet door.
Revealing mistake: When Natalie is in the old building and she is looking around, she opens the door and Parker's dead body falls out onto the floor. When it cuts to a close up of his body, you can clearly see the side of his neck pulsating although he is dead. (01:19:50)
Revealing mistake: When Ben falls from the roof and into the leaves during the storm, you can see the mat that he lands on move from under the leaves.
Revealing mistake: When one twin (presumably Alfred) is removing the other twin's fingers, "Alfred's" upper arm passes through his twin's head as "Alfred" reaches for the hammer. [Fixed on DVD.]
Revealing mistake: When young Ichi has a "climax" at the end, you can tell his "spunk" is really just white goop and isn't real.
Revealing mistake: When Laura falls in the bathtub, a stunt is doing the fall, because her hair colour and style changes.
Revealing mistake: When the killer is dragging the girl's body with the meat hook, you can easily see that it is a doll. (01:18:45)
Revealing mistake: The "hand" holding the gun rotates 180 degrees while the shooter's point of view does not move. A real hand can't reproduce that motion without moving extremely close to the face.
Revealing mistake: The dead woman who is found at the beginning of the movie takes a rather deep breath at the end of the shot. (00:02:40)
Revealing mistake: After Majid commits suicide, he is breathing.
Revealing mistake: In the shot of the helicopter hovering above the apartment building, the blue screen technique is needlessly revealed by the eratic and physically impossible movements of the chopper. This could have been avoided if the camera shooting the aircraft had been stationary. It appears, however, that it was a handheld shot.
Revealing mistake: As Hi is racing after the baby, a close-up of his tired face driving the brown Caprice shows the gear selector is in "Park".
Revealing mistake: When Jean Reno and Vincent Cassel approach the hanging body of Jean-Pierre Cassel in the doctor's house, the hooded killer can be seen standing to the left of the screen. [Not visible on the VHS version, but on the widescreen DVD].
Revealing mistake: In the scene where Sgt Howie hits McGregor (who is dressed as Punch) with a candlestick, watch closely and you can see that the blow from Howie is aimed into the padded section on the back of the punch costume, rather than at the back of McGregor's head.
Revealing mistake: When Hayden is talking to the Psychiatrist in the Hospital, he is wearing some type of white powder face makeup to make him appear more "ghostly". This makeup was only applied to his face. Whenever they show a side shot of his face and neck during this scene, you can see the demarcation (line) of where the white powder face makeup ends and his natural darker tan neck begins. (01:05:20)
Revealing mistake: The outline of the blood pack is visible under John's shirt when Stillson's body guard shoots him. (01:37:35)