Cal. Gov. Code § 59123

In any procedure taken pursuant to this chapter, if any neglect or omission of an officer of a local agency, or any error or informality, does not directly affect the jurisdiction of the legislative body to issue the assessment readjustment bonds, it does not invalidate the proceeding or any assessment for the cost of the proceedings. The exclusive remedy of any person so affected or aggrieved is by appeal to the legislative body pursuant to Article 9.