Corrected entry: In the beginning of the film Donnie and Lefty are towing a Porsche behind Donnie's Cadillac. They seem to have the Porsche attached to the Caddy with some rope or maybe a leather belt (way too close together to be a proper A-frame tow bar). The problem with this type of towing is that any time the Caddy would slow down or accelerate hard the Porsche would slam into the back of the Caddy.
Corrected entry: One scene shows Lefty and Donnie feeding the lion hamburgers through a crack in the window of the Cadillac so as not to be bitten or attacked by the lion while feeding it. It is never explained how they were able to drive a lion to a hamburger stand in the back seat of their car without being attacked by it.
Corrected entry: In the first scene, where Sonny Black and his crew are at a bar sitting around a table, they're arguing about cars. Lefty repeats that one car has more power than the other. When he repeats that, Paulie says "You just said that". The problem is Paulie began to say that right before Lefty finished saying that the car had more power. In this context, Paulie would need enough time to hear what Lefty said, register it in his brain and then respond. This shows that "Paulie" jumped the gun and began his line too early to be believable.
Corrected entry: In the scene where Sonny Black's crew is about to carve up Sonny Red's crew, Nicky grabs Donnie by the arm to help him cut up the fat guy. As he's walking towards the fat guy's body, Nicky says "Help me fillet this fat f**k". Right there he looks directly at the camera and then looks away quickly.
Corrected entry: In the scene where Depp and Pacino are crossing the bridge in the car, Depp is driving and you can see he has his wedding ring on. He has been undercover for a long time, I guess he forgot to take it off after his last shot. If Pacino had seen it, he would have been shark bait.
Corrected entry: In the scene where Sonny Black introduces Donnie Brasco to the Boss of Florida on the boat, he introduces Donnie as a "friend of ours". To call someone a "friend of ours" means that they are a member of the mafia, which at the time, Donnie Brasco was not. The correct phrase would have been a "friend of mine". The difference between 'friend of mine' and 'friend of ours', and it's inflexibility, is clearly explained by Lefty earlier in this movie.