Errol Morris's 1988 investigation of a Texas death-row case. The film's evidence freed Randall Adams from prison — the most consequential documentary ever made for its subject.
From Wikipedia
The Thin Blue Line is a 1988 American documentary film by Errol Morris, about the trial and conviction of Randall Dale Adams for the 1976 shooting of Dallas police officer Robert W. Wood. Morris became interested in the case while doing research for a film about Dr. James Grigson, a psychiatrist known in Texas as "Dr. Death" for testifying with "100 percent certainty" of a defendant's recidivism in many trials, including that of Randall Adams.