Came across this article today - not the same as imprisoning an innocent man, but still a huge miscarriage of justice nonetheless.
Your justice system at work
Cliff notes: Borderline retarded 16-year old indirectly (and perhaps unknowingly) involved in a quadruple homicide is still in jail 20 years later while one of the murderers is free.
edit: you need to register to read it, but it's worth it