This new Northwester University study claims to have identified the erroneous judgment rates of US juries (17%) and judges (12%) . The rates are higher for convicting an innocent person than for acquitting a guilty person. The article does not explain how the researchers evaluated the judgments, but the figures seem about right to me.
The article also fails to compare these rates to those in other countries, or to previous times in the US. My sense is that these rates are much better today than in years past, and better today in the US than in any non-democracy / civilized nation.