Continuity mistake: In the scene where Tom Cruise gets out of jail and has a confrontation with Jason Lee, you can clearly see (towards the end of the scene) that in Jason Lee's shot it is snowing, and in Tom Cruise's it is not, then when it goes back to Jason Lee it is snowing again. (01:32:45)
Continuity mistake: When the panel of doctors are being queried by Aames, for the second time of the loop of this interview, the black box containing the mask can clearly be seen in the bottom right hand corner on the desk when the camera swings to the main doctor. When the camera pans back again, it is gone, and it is only after this that the assistant brings in the box.
Continuity mistake: In the scene where Tom and Penelope are lying together on her sofa there is a table to the left with a lamp and some wooden artists figures. These move position between shots, with different things happening with their arms.
Continuity mistake: In a scene during David's birthday party there is a sequence showing Julie and Brian talking on a sofa with a blank white wall in the background. Later, in a different scene showing Julie dancing with one of the waiters holding a tray of martinis in front of the same sofa, there is a black and white picture of a car in the background.
Continuity mistake: In the scene with David's birthday party, he comes downstairs after being with Julie, and is talking with Sofia about how Julie is stalking him, when Julie is staring at them, her part in her hair goes from being on the left side to being on the right side, then back again.
Continuity mistake: In the scene where David discovers Julie in his bed her strap is on her arm. In the next shot it is where it should be.
Continuity mistake: When Tom takes Penelope home, it is dark outside. However, when they go inside seconds later, there is clearly light (and more than just streetlight) shining through the curtains, as if it were day.
Continuity mistake: When David and Sophia go upstairs to his friend's apartment, his friend walks in. When he is talking to David in this scene, in one shot David's hair is perfectly normal, and then there is a shot of his friend, but when it comes back to David his hair is abnormally messy and sticking up. In the next shot of David it's back to being flat again.
Continuity mistake: In the scene where Penelope is looking at the drawing of her face, she is holding it in her left hand. But next shot (camera behind Tom Cruise's back) shows that she has the drawing in her right hand.
Continuity mistake: When David comes back to Sophia's apartment, towards the end of the film, he rips the photos off the fridge. But in the next shot, when he is throwing something to the wall, the photos are still on the fridge.
Continuity mistake: Tom leaves the bar after wearing the mask in public for the first time. He and his best friend have a argument outside in an alley. As they cut to and from Tom his hair is hilariously out of whack on one of the shots.
Answer: It's from an Icelandic band called "Sigur Ros", and it is not on the soundtrack (although another one of their songs is - one from earlier in the movie). It's from their 2002 album titled "()" - yes two parenthesis, it's on track 4, but is an untitled song. You can get the album on Amazon.com or in any Borders or Barnes and Noble.