And check out the expose's regarding the case of Michael Morton here at Texas Monthly. I was particularly interested in Pamela Colloff's Why John Bradley lost that shows that the then active Williamson Country District Attorney, John Bradley, attempted to have DNA evidence destroyed that when admitted, ultimately led to Morton's exoneration. Fascinating and sordid tale of Lone Star justice.
...Bradley advocated seeking DNA destruction as part of plea agreements on the grounds that "Innocence ... has proven to trump most anything." "A better approach," he said, "might be to get a written agreement that all the evidence can be destroyed after the conviction and sentence. Then, there is nothing to test or retest."