Wat. Code, § 60048

No informality in any proceeding or informality in the conduct of any election, not substantially affecting adversely the legal rights of any citizen, shall be held to invalidate any incorporation, annexation, exclusion, or other action under this act. Any action or proceeding wherein the validity of any such incorporation, annexation, exclusion or disincorporation is denied or questioned, shall be commenced within three (3) months from the date of the certificate of incorporation, annexation or exclusion issued by the Secretary of State, or from the date of the order of the board of supervisors declaring the disincorporation, as the case may be; otherwise, said actions and/or proceedings in respect thereto shall be held to be valid, and in every respect legal and incontestable.