Or, as has come to light in this Texas case, the real killer remains free to kill more people. True, therewas willful suppression of exonerating evidence from the get-go, and it wasn't death penalty, but it does highlight that there is harm to far more than the convicted.

http://www.npr.org/2012/04/28/150996459/free-after-25-years-a-tale-of-murder-and-injustice