Plot hole: Shaft's friend has been killed at the beginning of the movie. Bad guys show up at his sister's door for a fake flower delivery, and Shaft kills them. He asks the dying baddie "Who put a contract out on me?", but how does he know? We as audience know there is indeed a contract on Shaft since we've seen the conversation with the mobster, but as far as Shaft knows, all evidence points at Arna being the target, and he even explained the money-related motive a mere minute before.
Continuity mistake: When Captain Bollin is questioning Shaft in his office, his hand position between shots is not always consistent. Best example when he asks Shaft about Mascola: he is leaning with his left hand on the top of the file cabinet, but he's shown rubbing his hands together in the following shot. (00:29:40)