Mont. Code Ann. § 85-7-2201

No suit, action, or proceeding shall be brought in any court in this state attacking the validity or regularity of any order or decree of the court purporting to establish a district hereunder or correcting or amending any order or decree creating a district after the expiration of 6 months from the date of the recording of the order establishing the district or the order correcting or amending the same. Nothing contained in this section shall be deemed or construed to confirm, approve, or validate any warrant issued, disbursement made, or contract entered into for the expenditure of money by the board of commissioners on behalf of any district.