According to a recent article in The Texas Tribune, the state of Texas comes in third behind Illinois and New York in the number of exonerations in the United States since 1989. This is according to data compiled by Mother Jones. You should read the full Texas Tribune article.
You should also read the story in Mother Jones to see how Texas Governor Rick Perry was forced to pardon an innocent man years after he had died in a Texas prison. The story of Timothy Cole and his struggles with the Texas criminal justice system is tragic indeed.