Continuity mistake: When Derek lifts the till to throw it through the window it's wired glass, when he throws it through it's obviously clear. (00:46:00)
Visible crew/equipment: When Cameron is talking to Derek in his office and says "You're on safe ground.", tripods, stage lights, and set parts are reflected in his glasses. (01:02:20)
Continuity mistake: When Derek is first telling the story about his time in the jail they show Derek with the top button on his shirt buttoned up and the next shot the cop tells him to button up his shirt and the top button was unbuttoned. (01:09:15)
Visible crew/equipment: When Derek comforts Danny in the bathroom after he has been shot, we see a shot of the detective. If you look behind him you can make out a pole of a boom mic. It isn't clear as the background is slightly blurry, but it's there. (01:53:45)
Continuity mistake: When Danny gets shot at the end of the film, his paper flies out of his hand. But the very next shot of him shows that the paper is still in his hand being held over his head while he falls backwards. In one of the following shots the paper is back on the floor, but it's on the opposite side it should have been when it flew out of his hand. It's all shown in slow motion and it's very obvious.
Continuity mistake: Toward the beginning, when the principal calls Danny into his office to discuss his Mein Kampf paper, he first glares at the Jewish teacher who exits the principal's office. As the teacher is walking past, the clock on the wall reads 8:10. But when the camera angle changes to show him walk around a corner, the same clock reads 8:20. Apparently it took him 10 minutes to turn the corner.
Continuity mistake: When Danny and Derek are taking down the Nazi-posters in Danny's room, the big poster above the computer is a very large Nazi flag that takes up about one fifth of the entire wall. During one of the shots in which all other smaller posters are being taken down, it is not there, and then it reappears and is the last one they take down.
Visible crew/equipment: In a scene in the principal's office reflections of the crew can be seen in the bookcase glass. A crewmember wearing headphones and holding the boom can be seen fairly clearly.
Continuity mistake: After the shooting, when Derek is being arrested, he is on his knees and the cop behind him helps him to his feet. The cop wraps his right arm across Derek's chest, the camera then cuts to the angle from Danny's view, and it's now the cop's left arm across Derek's chest when he gets to his feet. Then the 3rd shot shows his right arm across Derek's chest again, as he's being hauled away.
Continuity mistake: In the scene where Edward Norton is in jail and talking to that guy who does the laundry, Norton throws the towels in the rack. Every time the camera angle switches between Norton and the laundry guy, the towels are neatly folded, then crumpled, then folded again, then crumpled again.
Continuity mistake: During the scene where the skinheads raid the store, one of the store employees is on the floor being attacked by a few skinheads. They pull two liters of soda off the shelf hitting the employee. In the next scene they cut a bag of corn (or seed) that falls all over the employee but the two liters are now back on the shelf.
Visible crew/equipment: When Danny is talking to Cameron just before Derek enters the room, look at the map of the United States on the wall right behind where Danny is sitting. There is a visible reflection of a stage light. There are several other lamps in the room as part of the make up of the room, but due to the size and intensity of this reflection, it's quite obviously from a stage light.
Continuity mistake: Toward the end of the film, when Danny is approaching his girlfriend at their high school, his backpack is on his back. He then hugs her, and the bag is not there as there are no straps doing down the front of his shoulder. They are then shown walking together into the school and the backpack is on his back again.
Visible crew/equipment: Before the killing of the black guy happens, in the scene where Derek is arguing with the English Teacher (Murray) about civil rights, he hurts Jennifer Lien. He then takes off his shirt and shows Murray the German Swastika on his chest, as he does this you can easily see some of the undressed studio set behind him as instead of a ceiling there is simply a black wall at the top.
Continuity mistake: When Derek exits his house and shoots at the three black guys who are stealing his truck, there are numerous street lights lighting up the street throughout this scene. Then during the slow motion shot where Derek is approaching the guy who he gives the Californian curb kick, the lights are out, and the entire street is dark. All the lights apparently must have simultaneously turned off.
Continuity mistake: When the three African-American guys throw the white guy into the bathroom to beat him up, watch the guy who locks the bathroom door. In one shot he is several feet behind his two friends, with his back to the camera locking the door and in the next shot he is standing right there with them.
Continuity mistake: At the dinner table scene at the Vinyard home where Derek and Murray get into an argument about civil rights, watch Derek's arms right after Murray says "It's like an expression of rage by people who feel neglected and turned away by the system." He moves his right arm back behind his plate, then in the next shot he has this arm extended forward holding his fork.
Continuity mistake: Just after the black guy shoots Danny at the end of the film, blood spatters all over the wall behind him. In an immediate next shot when Danny is falling backward, the blood spattered is in a noticeably different pattern.
Continuity mistake: In the opening scene where Derek is in his prison cell and the warden is waking the inmates up in the morning, every prison cell door is showed simultaneously opening. Seconds later, in a following shot Derek is shown exiting his cell and his cell door is still opening after every other cell door was shown completely opening.
Continuity mistake: When they are playing the basketball game and Seth bets that he can beat the black gang, he has sweat all over the collar area of his t-shirt. But in the next shot and for the rest of the game, the sweat is gone.
Answer: It's quite simple: He was one of them, a fellow skinhead. In prison different groups stick together for mutual protection and comradeship, and as neo-nazis/skinheads would be fairly unpopular with the colored inmates, all the more reason for them to protect each other.
Twotall