Mistakes happen. If a mistake occurs in the property tax process, it could be costly if not corrected. Fortunately, some errors are fixable -- even those that may have occurred in a previous year or years -- thanks to what is known as a Certificate of Error, or in property tax parlance, a C of E. When an assessment error is discovered, taxpayers can seek relief by filing a C of E with local assessing officials. However, be advised that the granting of a C of E by an assessing authority is discretionary, not mandatory.