One of the strengths of the Law And Order shows, especially the prime show is that it was never afraid to show that the police and prosecution were never perfect. They made errors like the rest of us mortals. Only their mistakes could really jam people up.
Detectives Jerry Orbach and Benjamin Bratt zero in on a witness to the events prior to the crime instead of the crime itself as the perpetrator. Just prior to the shooting of a young man, Frank John Hughes helped him change a tire and as he left he saw the eventual perpetrator show up. His 'crime' was going into an all night coffee and donut shop and lifting a bunch of napkins to clean off the grease on his hands.
I won't go into all the details, but later on when Briscoe and Curtis are focused on the right suspect Hughes is reluctant to cooperate. When you see the episode you won't really blame him. Hughes is scheduled to be married and his future father-in-law John Doman who is a housing police cop acts like his attorney. Maybe Doman once had a wrong focus on a case and got burned with it. In any event Sam Waterston's case looks like it's going in the toilet unless Hughes cooperates.
I've seen this happen a few times in my days at Crime Victims Board. Cops are in real life far more reluctant than Briscoe and Curtis to abandon original theories on a case.
This episode is a case study in why sometimes people are reluctant to be good citizens.