In other words, it takes absolute certainty to go to work every day and send people to prison for up to a lifetime. And certainty is tricky to surrender. Allowing for the probability that you were wrong once allows for the possibility that you have been wrong often, and opening that door is often unthinkable.

Scott Cupp has seen how prosecutors learn to shut the mental gates against doubt. He was a prosecutor in Florida for 17 years. “I put a lot of folks in prison,” he said, adding, “but before I did I made damn sure I was right.” Several times during his tenure, he said, he was presented evidence that he had the wrong “guy” and faced reluctance among some of his prosecutors to drop the charges, “but we ended up doing so — it’s called doing your job,” he said.

Then Mr. Cupp became a defense lawyer. In fact, he was Mr. Schofield’s lawyer off and on until he was offered a position as a judge on the 20th Judicial Circuit Court in Charlotte County, Fla., in 2014. The case haunted him while on the bench, and when he heard Gilbert King speak at an event in Naples, Fla., in 2018, he told the reporter to go read the case transcript because his client was “not just wrongly convicted. He’s an innocent man.” In March, Mr. Cupp stepped down from his position on the court to defend Mr. Schofield again, full-time and free.

“The system is made up of human beings, and human beings are flawed,” he said. “In this case, you add a relatively small town where law enforcement and prosecution is a big fish in a little pond, it’s ego pure and simple.”

When there is progress in such “small ponds,” therefore, it can be incremental. Hints of such an evolution, however glacial, were visible in the room at the latest Schofield parole hearing Mr. Cupp said. (Parole has been abolished in Florida, one of 16 states to do so, but because that happened after Mr. Schofield was sentenced, he is still entitled to parole.) The room was filled with journalists and sympathetic spectators.