No informality in any proceeding or in the conduct of the election, not substantially affecting adversely the legal rights of any citizen, shall be held to invalidate the formation of the district. Any proceedings wherein the validity of formation is denied shall be commenced within 90 days from the date of filing the order declaring the result of the election with the Secretary of State, otherwise the formation and the legal existence of the district shall be held to be valid and in every respect legal and incontestable.
Source:
Section 100049, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=PUC§ionNum=100049..