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.
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)
Visible crew/equipment: When Derek is running into the bathroom after Danny has been shot, as he is running through the crowd of students, it cuts to a shot from the side of the hallway, it's very quick, but you can see a camera, a cameraman, and the boom mike and its operator.
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)
Audio problem: At Cameron's party, when Derek is about to leave and Seth comes at him with a gun you can see Danny running down the stairs screaming at Seth, but his mouth doesn't move.
Answer: The tattooed DOC on gang members' arms means Disciples of Christ - the gang that they belong to in the movie.