OK, I'll give you 2 sources. One you'll hate, because it does come from activists - but that does not change the fact situation. Here is a list of a bunch of people wrongly convicted, whose sentences were later over turned with new DNA evidence:
http://www.innocenceproject.org/case...ar_exoneration
The second, you can look up yourself, state by state, nation by nation - but every time someone has a case overturned on appeal, it's a wrongful conviction. There are varying degrees - some of these people are 100% innocent, some perhaps were not - but the conviction is overturned. Here are some British stats:
http://www.radstats.org.uk/no076/naughtont1.htm
I assume the Justice Department at state or federal levels has similar info.
In Canada, we've had 3 high profile cases - David Milgaard, Guy Paul Morin and Donald Marshall - all convicted of murder, all would have faced the death penalty had our nation still used that punishment, all were freed after years of imprisonment and/or numerous trials. I'm glad no one put a bullet in these poor buggers.