The governor of Illinois suspended the death penalty after finding about half of the convictions were unsafe. One reason is this: the death penalty selects jurors more likely to convict. You cannot be on a jury for a capital case if you don't support the death penalty. So this selects for more 'conservative' jurors. I don't mean all conservatives always believe everything the police say - but they are MORE LIKELY to. This creates an inbuilt bias in favor of conviction.

The governor of Texas (one of the Bush klan) said at the time that Texas had no such problem - all of Texas's capital convictions were safe as houses. Turns out they are about as safe as trailers in a Texas tornado:

http://www.guardian.co.uk/world/2012/may/15/carlos-texas-innocent-man-death?newsfeed=true