West of Memphis (2012)

This is a documentary about a miscarriage of justice that took three previous films, a lot of famous people, and a team of investigators to get to the point where an injustice was reversed. 
In 1993, the bodies of three young boys were found bound, mutilated and drowned in a drainage canal in the Robin Hood Hills neighborhood of West Memphis. A month later, three local teenagers were linked to the crime after one of them confessed; the prosecution claimed it was a satanic cult murder.  The community seized upon this, not paying much attention to how the confession was elicited or who made it, and as a result, a guilty verdict was achieved and nobody else was closely investigated.
This film does that.  They get a team of forensic pathologists to look at the evidence, there is new DNA information, and there is one suspect identified who was never looked at very closely, maybe because he had what turns out to be a largely fabricated alibi and perhaps a sympathetic police force.  The ending is not as satisfying as you would hope but better than might have otherwise happened, and it demonstrates that if you get it wrong the first time, it is very hard to undo that in the criminal justice system.