The system works almost always but does get it wrong sometimes. I doubt that this was that time. Justice was served.
Not really - the legal discovery rules require full disclosure of all evidence, yet you regularly hear about how prosecutors don't disclose evidence. Sometimes the appeals process will grant a new trial, sometimes they overturn, but often the judges will just say "we're not going to bother worrying about the lack of a fair trial". It happens often enough that prosecutors are willing to take the gamble on it, otherwise it wouldn't continue to be a common news story.
Read the background here and tell me you really support this man who stabbed that woman - mother and wife - 43 times.
https://law.justia.com/cases/missouri/supreme-court/2003/sc-...