"Volunteer Protection: The Volunteer Protection Act generally protects unpaid volunteers from liability for acts or omissions that occur while acting within the scope of their responsibilities at the time of the act or omission. It provides no protection for harm caused by willful or criminal misconduct, gross negligence, reckless misconduct, or a conscious, blatant disregard of the rights or safety of the individual harmed by the volunteer."

And any taxpayer you help can haul you into court for a judge to decide whether you met these terms.  You may not be liable for damages, but you're liable to spend a day or more at the courthouse.  

We have some volunteer sites in retirement communities where preparers enjoy helping people with complicated returns, because if you follow the VITA qualification tests you are supposed to limit your work to easy returns for low-income filers.  Maybe they do a better job of screening now, than when I was accepting new clients and some of them were VITA refugees.