this group:
http://www.innocenceproject.org/
has worked to examine many cases in the US of people convicted of murder, many on death row awaiting their execution.
In the 16 years since they began their work, they have helped free some of these people by proving that they simply couldn't have done it.
They have saved the lives of not 10 innocents, not 20, not 100, but 218. That's 218 people who could have been put to death for crime they didnt commit - and 218 cases where the real killers were free to potentially kill again.
That's in one country alone, and they only manage to check a handful of the cases that land on their desks.