This extensive investigation by KSAT ABC Channel 12 delves into the use of unreliable drug informants, planted drugs, lack of supervision, and a host of other debacles that led to the wrongful conviction of multiple people in San Antonio, Bexar County, Texas. You can watch the hour-long special here; for additional videos, interviews and resources, check out their Confidential Informant page.
This kind of large scale drug scandal happens more frequently than you might think. See these previous posts for additional examples in Florida, Tennessee, and Texas.