Ten of the eleven exonerations in Texas so far this year are drug possession cases in Harris County. These are cases where defendants pled guilty before the alleged "drugs" were tested. Testing later proved that the substances in their possession at the time of their arrest were not drugs.
So why did these people plead guilty? One possibility is that they thought they were drugs. In this scenario, a drug dealer likely sold them bogus product.
Another possibility is that the defendants thought it was the quickest way to resolve the case. Plead guilty, do a little time, get on with your life.
Either way, it's still bad justice.
The Harris County district attorney's office is, as we understand it, no longer moving to convict in similar cases before making sure that the drugs are real. This is what we call common sense. :)