The accuracy of the E-Verify program has always been argued, but recent statistics released from USCIS show that this is a very accurate system. The statistics are based on cases submitted through E-Verify between October 2009 and September 2010, the most recent fiscal year for USCIS. Out of over fifteen and a half million cases 98.3% of employees were automatically confirmed. Of the 1.7 % of employees receiving system mismatches, 0.3% contested and resolved the mismatch and 1.4% were not found work authorized.

This puts serious doubt to the argument that there are up to 20% of work authorized employees having to contest and resolve errors stemming from the E-Verify system. I also find it interesting that only 1.4% are not work authorized, which means that either the number of illegal immigrants working in the US is significantly lower than what has been reported or illegal workers are applying at other employers that are not doing the verifications. I tend to believe it is the latter. 

USCIS also reported that through regular updates and enhancements, the E-Verify system continues to improve its accuracy.