Continuity mistake: When Danny gets home from school on the day of Derek's return, he shows Derek his Disciples of Christ tattoo on his arm. Watch Derek and you will notice that he first grabs Danny's wrist with his right hand, and then in the immediate following shot of completely uninterrupted time Derek is holding Danny's wrist with his left hand.
Continuity mistake: Watch behind Danny when the principal tells him to shut the door to his office. Right before he says this, on the wall behind Danny, there is no reflection of the sun. When the shot cuts back to Danny we can see a large reflection behind him on the wall from the sun.
Continuity mistake: When Seth is spitting the jelly beans out of his mouth, he has them all over his shirt. But in the next shot when Derek comes into the room and Seth gets up, there are none on his shirt.
Continuity mistake: When Derek is talking to Danny in the kitchen about the stunt he pulled in school, the lamp above the kitchen table goes from being still to moving side to side between shots.
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)
Continuity mistake: After Derek stamps the car-jacker's head into the curb it shows the dead body with his head on the curb. It cuts to Derek, Danny, police then goes back to the dead body. His head has somehow slipped off the curb and is now behind it.
Continuity mistake: When Danny is walking into the principal's office he is not quite in the door when the shot goes to Sweeney and he says to 'shut the door', then the shot goes back to Danny who is about two feet farther in the door than he was when he stopped in the last shot.
Continuity mistake: When they go into the coffee shop they order a coffee, a large milk, and a maple bar. Then when Sweeney and the cop walk in Derek goes to talk with them without taking the order. He tells them that he is going to take him to school and calls Danny over. Danny doesn't come close to the counter but when they walk out the door Danny has the order in his hand.
Continuity mistake: In the first basketball game in Venice Beach, Edward Furlong leans up on a fence and his school bag's strap slips off his shoulder. In the next shot, the strap is back up on his shoulder. Yet he had no time to fix it.
Continuity mistake: In the dinner scene with the whole family and the mother's new date they are talking about the LA riots when Edward Norton puts down his fork. It cuts to a shot behind him and he's still holding the fork up.
Continuity mistake: When Edward Furlong is called into the principal's office in the beginning of the film he picks up his school bag with his right arm, and then enters the office with it on his left arm. And there is no time in between for him to change arms.
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)
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