Continuity mistake: After Frank commandeers the driving instructor's car for a chase, the woman learning to drive is faced with an oncoming truck. In one shot, the instructor has glass all over him, but in the next shot of him, the glass is all gone. (00:33:30)
Continuity mistake: At the end of the movie, when Jane is under control, she points the gun at Frank. The shot behind Jane, she has both hands on the gun. It is tough to see, but it is happening. The camera changes to show Jane, and her left arm is completely down at her side.
Continuity mistake: When Frank's plane is landing, it's a DC-10, but when Frank is getting off the plane, it's an L-1011.
Continuity mistake: When Frank hits the garbage cans in his car, causing the airbag to go off, watch how they are inflated during this sequence. The airbags are deflating in the oncoming angle but then are sticking out of the passenger window in the opposite angle.
Answer: Because Nordberg was working undercover in Ludwig's organization; not knowing this, the snitch would only see him working with Ludwig's crew, and would thus believe he was dirty.