Mo. Rev. Stat. § 243.500: Validity of bonds issued unquestionable.

Hereafter when any bonds issued by any drainage district organized or incorporated under the laws of this state are sold for a sum or price allowed by law, and the proceeds of the sale thereof paid to the proper officer of the district, then except for the power to issue the bonds, neither the validity of them nor the tax levied to pay them shall thereafter be questioned in any court of law or equity.