I haven't seen it, but I know the gist of it.
Just yesterday, CNN rebroadcast a documentary type story they did on a guy in Texas who spent 25 yrs in prison for killing his wife. The prosecutor (who had gone on to become a judge) had withheld crucial evidence that would have likely exonerated the guy, if put before a jury. Even worse, it was evidence that could have likely lead police to the real killer, who we now know killed at least one other woman about a year later.
The DA had fought tooth and nail for over 6 yrs to avoid having a bandanna left about 100 ft away from the house of the victim, which appeared to have blood and hair attached tested for DNA. It was through DNA analysis, that showed the bandanna carried the blood of both the victim and the perp and removed all doubt about the innocence of the poor sap that spent 25 yrs in prison for something he didn't do.
http://www.cnn.com/2013/12/04/justice/exonerated-prisoner-update-michael-morton/
In all, between 1989 and 2012 at least 2,000+ innocent people have been exonerated after being convicted and held in prison for crimes they didn't commit. The number of actual innocents held in prison is almost certainly a hell of a lot higher, given that in many instances those items in evidence that can be tested for DNA/Blood evidence are often destroyed or "lost" in the years following their conviction. Again, the prosecutor and DA were sitting on evidence that could have revealed the identity of the actual killer and they fought for 6+ yrs to deny the guy's defense team the right to have the DNA testing done.
Bottom line (IMHO) there's a hell of a lot more lazy, incompetent and downright corrupt types in LE and the Prosecutor's office than most would likely believe. I had already changed my mind about capital punishment during my time in law school, but attending a private screening of the documentary A Thin Blue Line and meeting Randall Dale Adams, the subject of the movie who was another person wrongfully convicted, of the murder of a police officer in Dallas. His conviction was secured in large part by the DA's usage of perjured testimony (which he had to have known was perjured) and the withholding of crucial evidence, just cemented my opposition to the death penalty. In Adam's case, the DA was running for higher office and wanted the PR that would come from convicting a "cop killer." (Even when he almost certainly knew, the actual killer was a 16 yr old kid, who had given the hitch-hiking Adams a ride earlier in the day and provided the perjured testimony against Adams. But because he was a juvenile and wasn't subject to the death penalty, the DA basically framed the adult Adams to get the most widespread publicity he could.)