The Treasury Inspector General for Tax Administration yesterday released Ensuring the Quality Review Process Is Consistently Followed Remains a Problem for the Volunteer Program (2012-40-088):

The Volunteer Program provides no‑cost Federal tax return preparation and electronic filing to underserved segments of individual taxpayers, including low‑ to moderate‑income, elderly, disabled, and limited‑English‑proficient taxpayers. However, preparing accurate tax returns remains a challenge for the Volunteer Program. … The accuracy rates of tax returns prepared for our auditors at IRS Volunteer Program sites increased from 39% in the 2011 Filing Season to 49% in the 2012 Filing Season. Three of 16 tax topics tested contributed the most errors—the deductions for Educator Expenses, IRA contributions, and small business expenses.

http://taxprof.typepad.com/taxprof_blog/2012/09/tigta-tax-returns.html