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.
Factual error: There is one scene in the movie where Heath Ledger reads a Latin sentence in a book. As he is translating the sentence, he goes over the words one by one with his finger. This would never be possible in a Latin sentence, because words in Latin are never - I repeat: never - in exactly the same order as in English. Words that actually belong together can be placed on opposite ends of a sentence.
Audio problem: When Steve is thanking Melissa for marrying Mark, the bride turns out to be Angela. Angela knocks Steve to the floor and grabs a gun from under her wedding dress. You can hear the sound of a hammer cocking but the gun turns out to be a hammerless automatic.
Continuity mistake: Curtis is helping Molly seal the windows by screwing them shut. He hands the drill to Molly and she holds it up by her chest. When Claudia walks into the room Molly lowers the drill down in front of her below her waist. Then there is a quick cut back to Molly and Curtis as he says, " well that should do it", and the drill is back up in front of her chest again as she lowers it down a second time.
Continuity mistake: When John comes to get Dawn and they leave Sally's house, the Chihuahua isn't with them, but all of a sudden it is back with them on and off throughout the movie. (00:05:45)
Visible crew/equipment: When R enters the control room where Cyper is waiting and the camera moves towards Cyon, you can see the reflection of the camera in the curved glass. (01:35:20)
Continuity mistake: This is actually in a deleted scene on the DVD: Jones shoots a mountain lion at night, and it's seen to be completely dark out. The two other men go out to look at it, but when they're walking across the snow, the sky is suddenly pale blue and there's light just below the horizon. It looks like it's just before dawn or just after sunset. But when they reach the dead mountain lion, the sky is suddenly black again, and it looks like it's the middle of the night.
Other mistake: Sam asks Leo if she can see his gun, then she points it at him. He laughs and says the clip is removed and that he would never have let her have a loaded gun. She shows him the clip she has stolen from his pocket, loads it into the gun, and cocks it. When she cocks it, a round comes out, meaning that there was already a round in the chamber, so he did give her a loaded gun. She only had one shot, but it was still loaded.
Factual error: The film is set in the 1950s. In the scene where Ewan McGregor and Peter Mullan are waiting for a load of coal (the morning after they discover the body), there is a close-up shot of Ewan McGregor's head and shoulders. In the background is a huge container ship, slightly out of focus. Containers were not introduced to Europe until 1966 and the ship in this film was probably built in the 1980s.
Factual error: At the beginning of the movie when Anthony Hopkins' character points out to the academic board the definition of "spooks" in the dictionary, it seems to be too close to the front of the dictionary to be in the S section. (00:07:10)
Factual error: Sean is tailing his victim, taking photos with an old Polaroid camera (the kind that prints instant photos). In one scene, he is sitting on a bench and takes several shots. When he shows these pictures later, some are zoomed in on the subjects, when it would not be possible to zoom with this camera. He would have had to get up and walk toward them, thus giving himself away.
Continuity mistake: When Colm Meaney and Colin Farrell have the shoot-out right at the end, the material filmed changes between then and when the director is viewing it at the end. First, Farrell shoots Meaney, then the camera goes to white and two more gun-shots go off. Then, when he is watching it later, Farrell pulls the gun from his pocket and the director shoots him straight away. Then when he rewinds it, it shows Colin Farrell pull his gun, the camera man look to the bonnett of the car where the gun is, then bring it up and shoot him.
Revealing mistake: When Maggie is chasing after the truck in the light aircraft what appears to be a "pasted-on" face in the cockpit, on closer inspection (with zoom feature on DVD players) is a male pilot flying the plane.
Visible crew/equipment: When Ben is in the fish and chip shop and is calling Judith Lucy, when the camera pans by one of the chip heaters, a boom mic is visible bobbing around on the reflection of the glass at the top just below the silver bar. (01:05:25)
Continuity mistake: After Candy is told about the internet video, Fred puts her blindfold back on. Her hair, which was hanging down in her face, is now pressed tightly against her eyes under the blindfold. The camera angle then changes and now we see a close up of Candy and her face (with the blindfold still on) is instantly hair free, brushed back neatly behind her ears instead.
Deliberate mistake: After Emily talks to inspector Girotti she starts a record player. Then she walks over to the window, sees Werner in the garden, and decides to go down. Between shots she appears behind him, which is wrong, as the song from the record goes on, but it's most probably intended. (00:30:45)
Visible crew/equipment: On the DVD, in the ending where Julianne looks over to the island after hearing a gunshot, then walks away, you can see the shadow of someone walking by behind the window of the cabin.
Other mistake: When "the killer" is pumping gas, if you count the clicks associated with a liter going into the tank, they equal a total of 33 liters of gas, not 27 which is what the pump stops on.
Continuity mistake: When Tom plays checkers with Bill and makes his last, winning move, there's a cut to a wide angle where the pieces on the board are arranged differently. (00:08:20)
Deliberate mistake: When the teacher walks over the floor towards the blackboard, he walks through a lot of blood. However when it cuts, he leaves no traces of blood where he steps.