Continuity mistake: When the drug tester gets done with testing the heroin, he blows out the lit candle on the bottom. But when there is a far shot of him talking, the candle is still burning.
Continuity mistake: During the car chase scene under the "B" Elevated Train in Brooklyn (86th St.), Popeye passes the same curve at least 3 or 4 times. Look closely, you'll probably see the same movie theatre over and over again. Apparently, they rerun the same chase clip to increase the length of this scene.
Factual error: The "B" train which he is chasing normally runs on the right most track (there are 3 tracks on this particular line). The middle one is never used except for out of service trains. Most of the train stations on that line service the right or left track, not the middle one.
Audio problem: When Buddy is in Sal & Angie's asking Angie Boca to model for him, their lips rarely match their words. Sometimes they appear to be saying something else, and other times it looks like they aren't speaking at all.
Factual error: Check out the sign over the front of the train, it should read "B", not "N". The "N" train runs on an entirely different line.
Factual error: In the beginning of the movie when the Frenchman is shot in the face, his face is instantly a mess of blood. In reality, an entrance wound from a pistol shot is a neat little hole (i.e. blood splatters from the bullet's exit wound, not the entrance). It would take awhile for blood to cover his face.
Audio problem: As Popeye Doyle is chasing the elevated train, he is continually honking his car's horn to warn pedestrians and whoever else to get out of his way. One scene shows him with one hand firmly on the wheel, and he hits the top-most rim of the wheel in frustration, and the horn honks on both hits. That car did not have a rim-blow type horn mechanism - only Fords had those, and this was a Pontiac Le Mans.
Continuity mistake: During the car crash scene, the Commanding Officer, Popeye, Detective Russo and the other cop are walking toward the accident. Close shot of Popeye, as he says, "I say we keep sitting on Boca", and they all stop walking. Immediate cut to long shot, and they are all still walking toward the accident.
Continuity mistake: At the start of the movie in France, we see an undercover French policeman shot by Pierre Nicoli, if you look quickly when the policeman is shot, we can see blood splatter all over his face. However, when we see the wider shot of him falling down his face is clean and has no blood on it at all.