Wash. Rev. Code § 85.08.520
If upon the foreclosure of the assessment upon any property the same shall not sell for enough to pay the assessment against it, or if any property assessed was not subject to assessment, or if any assessment made shall have been eliminated by foreclosure of a tax lien or made void in any other manner, the board of county commissioners shall cause a supplemental assessment to be made on the property benefited by the improvement, including property upon which any assessment shall have been so eliminated or made void, and against the county, cities and towns chargeable therewith in the manner provided for the original assessment, to cover the deficiency so caused in the original assessment.
If by inadvertence or for any cause the assessment levied shall be found to be insufficient to meet the entire cost of construction, a supplemental assessment shall be made by the board of county commissioners upon the lands of the district in the same proportion as the original assessment is levied, same being spread over not to exceed three years as the commissioners may determine.
Duplicate assessments or other errors that may by inadvertence be found to have been incorporated in the assessment roll may be corrected by order of the county commissioners upon same being certified to them by the treasurer and the engineer.
RCW 85.08.520