Neb. Rev. Stat. § 18-2478

For a period of thirty days after such publication any person in interest shall have the right to contest the legality of such resolution or proceeding or any bonds which may be authorized thereby, any provisions made for the security and payment of such bonds, any contract of purchase, sale, or lease, or any contract for the supply of water, power or electricity, energy conservation services or devices, or acquisition of energy sources or fuel, or sewerage or solid waste disposal services, and after such time no one shall have any cause of action to contest the regularity, formality, or legality thereof for any cause whatsoever.