Cal. SHC § 25375

The issuance of any bonds by the district shall be conclusive evidence of the regularity, validity and legal sufficiency of all proceedings, acts and determinations in any wise pertaining thereto. After the bonds are issued no assessment tax or tax or other charge levied or collected for the purpose of paying the principal or interest thereon shall be held to be invalid or illegal or set aside or refunded, by reason of any error, informality, irregularity, omission or defect in any of the proceedings, acts or determinations in any wise pertaining to the issuance or payment of the bonds. No action or proceeding shall be thereafter commenced or maintained to cancel or set aside the bonds or to prevent the payment thereof or the levy, collection or enforcement of any assessment tax, tax or other charge made for such payment.