Claire Williams thought her Mecklenburg County property taxes on the Steele Creek patio home she bought in September 2012 were paid up.

But late last month, she opened a $308 county tax bill that said the property had been undervalued during the mistake-riddled 2011 revaluation and that she owed back taxes for 2011 and several months in 2012 – when she didn't even own her home....