A police detective tells Johnny that he found is pistol in his hotel room with "one bullet fired". It is impossible to tell how many rounds have been fired from a semi-automatic pistol, without knowing how many had been loaded into it in the first place.
The fluorescent markings on the pavement as seen from inside the police van are obviously a photographic effect as the marks move around on the pavement and are appearing at a different pace than the van is traveling.
In scenes set in a pawn shop and an all-night diner (apparently shot in interiors on standing street sets), actors' frosty breath can be seen even though they're supposed to be inside real businesses that presumably would have been heated.
As the street cleaner rounds a corner in a nighttime street scene, the shadow of the cameraman and his equipment riding on a hoist is cast on the street cleaner.
Although the film is set in NYC, the street outside the hospital where the climax of the film takes place is lined with palm trees.