Serpico was shot in his face and operated on; when he wakes up after the operation, his beard is intact. The Emergency Room personnel would have shaved it off to treat the injuries caused by his being shot in the cheek.
In the opening sequence, when Serpico has been shot, a police cruiser rushes him to the hospital in heavy rain. Towards the end of the movie, when the sequence of events picks up again, it is not raining.
In the opening scenes, with the patrol car carrying Serpico to the hospital, the unit's emergency light has three red bulbs and one amber bulb as it spins. As the unit arrives at the hospital and they remove him from the back seat, the light has four red bulbs.
After throwing Corsaro against the wall in the squad room, Serpico begins searching him in front of his fellow coworkers. Just before he starts to search Corsaro's back pockets, the suspect's pants are still up and unblemished. The camera cuts away, then cuts back to the suspect's pants already torn with his buttcrack exposed before Serpico even starts his search. We then see Serpico start to rip and tear down Corsaro's slacks and drawers, thus exposing his crack.
When Serpico is in Lewishon Stadium (the amphitheater) talking with Bob Blair about going to the mayor, Blair (Tony Roberts) gets up and has an envelope in one hand and a newspaper in the other. In the next shot the envelope is gone, only to reappear from the newspaper while Blair and Serpico walk out.
The prison "chain gang" being led into the wagon at the beginning has male and female prisoners on the same "chain," and both sexes are transported in the same wagon. The NYPD absolutely forbade that then--and still does.
At the apartment raid, Serpico is shown armed with an automatic sidearm, possibly a 9mm. Serpico has said he was actually carrying his trusty Smith & Wesson snub-nose .38 special. It was a favorite of plainclothes and undercover officers because it is easy to conceal and draw.
In the beginning, at the hospital after Serpico had been shot, the NYPD lieutenant salutes Chief Sidney Green when Green is dressed in business attire. NYPD members only salute superior officers in uniform.
In the closing credits, actor James Tolkan's last name is misspelled as "Tolkin." Similarly, a police character in the movie with the last name of "Tolken" is also misspelled as "Tolkin."
Serpico's mother, an Italian immigrant, is depicted as understanding and speaking absolutely no English whatsoever. At the hospital following Serpico's shooting, the doctor speaks slowly, clearly, and deliberately to explain Serpico's condition to his parents, yet Serpico's father has to translate literally every single word to Serpico's mother in Italian. Serpico was 35 at the time, so his mother had to have lived in the United States for well over 35 years, at least. It is impossible that she could understand absolutely zero English after residing in the States for over 35 years.
When Serpico is being rushed to the hospital in a police car at the very beginning, the uniformed officer's collar brass says "73," indicating they are from the 73rd Precinct. When the car reaches hospital, it has "82nd Precinct" painted on its doors.
Serpico is on a rooftop just before he is shot, and the World Trade Center Towers are visible in the background. Serpico was shot during a drug arrest on February 3, 1971. The second tower was completed in January, 1972.
In 1960, as Frank Serpico drives across the bridge in his Studebaker Lark to report for his first posting as a patrolman, many of the cars in the background are from the mid-1960s to early 1970s.
In the latter part of the film Serpico is shown practicing with his new Browning Hi-Power at the range. He fires a total of 15 rounds at the target. This would be correct, if the magazine held 14 "shots", as the salesman tells him when he purchases it. In fact, the Hi-Power's magazine holds only 13 rounds, with one in the chamber, making a maximum of 14 rounds to discharge.
Serpico and Lombardo rapidly descend a shaky metal fire escape system without making a sound or alerting the lookout man, who only notices them when they fall to the pavement beside him.
When Leslie tells Serpico that she loves him at the party, her lips aren't moving.