Continuity mistake: At the beginning of the film in Brooklyn, it is obviously dark outside (nighttime). As they are chasing the guy from the bar, it all of a sudden is during the daytime as it is light outside.
Suggested correction: I think that it looks dark because they are under an elevated train track.
Other mistake: The French hitman hijacks the subway train and orders the motorman not to stop at any station. But why? He has no idea that Popeye Doyle is doing something as quixotic as chasing the train by car.
Suggested correction: This is not necessarily an error. The hitman may have anticipated that Popeye would radio ahead to the next station to have officers arrest him when he emerged from the train or may have thought that stopping at the next station would provide Popeye with time to catch up to him while the train was stopped at the station. If the hitman's intention is to put as much distance between himself and Popeye as possible, doing what is necessary so the train does not stop at stations is plausible.