When Billy Score shoots his first victims his gun has a silencer mounted on the barrel. We still hear "boom, boom" anyway. One shot is even heard from a POV outside the apartment where the killings are occurring.
Watch the locomotives pulling the train in the open scenes. Sometimes they're F.E.C. (Florida East Coast R.R.) locomotives and at other times they're Family Lines/Seaboard Coast Line/CSX. These are two separate rail lines to Miami that run parallel to each other but are a few miles apart. Much of the opening scene footage, however, seems to have been shot along the CSX (ex-Family Lines, ex-Seaboard Coast Line, ex-Seaboard Air Line) track.
When the killer shoots the shotgun through the door, the hole is jagged and oddly shaped, as it would be. Later, when the detectives are at the crime scene, the hole is perfectly round with smooth edges.
On the boat, Smiley tells Sharky, "When you went to see the man, you really pissed him off... you should have just turned[Dominoe] in. She'd be dead, but Nosh, Jo Jo and all your friends would be alive." But the way the movie is edited, Nosh and Jo Jo were killed before Sharkey went to see Victor to tell him Dominoe is still alive.