Mont. Code Ann. § 15-17-124

If the county treasurer discovers, prior to attachment of the tax lien, that property on which the taxes are delinquent has been irregularly assessed, the county treasurer may not attach a tax lien. The taxes on the property must be listed on the property tax record as uncollected for the year in which they were due, and they must be assessed and collected during the succeeding year as taxes are regularly assessed and collected.