Wis. Stat. § 75.22
If after the issuance of a tax certificate or conveyance to the county of any lands subject to a tax certificate and within the time hereinafter prescribed it shall be discovered that the certificate was invalid, the county board shall make an order, briefly stating the reason therefor, directing that the certificate, as it applies to the affected lands, or deed be canceled. But no certificate or conveyance shall be deemed invalid within the meaning of this section by reason of any mistake or irregularity in any of the tax proceedings not affecting the groundwork of the tax; nor shall any county be liable to pay or refund any moneys by reason of any such mistake or irregularity.
Wis. Stat. § 75.22
A defect in the groundwork of the tax means a defect or irregularity that necessarily affects the principle of the tax and shows that it must be unjust and unequal or an illegality or irregularity that results in an inequitable burden. In order for a county to argue that it has the authority to set aside a tax deed under this section, it would be required to show the property was inequitably taxed resulting in the property owner paying more than its equitable share of the tax. Jackson County v. DNR, 2006 WI 96, 293 Wis. 2d 497, 717 N.W.2d 713, 04-2582.