California Code, Health and Safety Code - HSC § 4809.2

No irregularities or informalities in conducting such election shall invalidate the same, if the election shall have otherwise been fairly conducted. Any action or proceedings, wherein the validity of the formation of the improvement district or of any bonds issued for it or of the proceedings in relation thereto is contested, questioned or denied, shall be commenced within three months from the date of such election; otherwise, said bonds and all proceedings in relation thereto, including the formation of the improvement district, shall be held to be valid and in every respect legal and incontestable.