Validity of tax proceedings preserved despite procedural defects when substantive action conforms to statutory intent and has been acted upon. Section 160 protects assessments and other tax proceedings from invalidation due to mistakes, defects or omissions if, in substance and effect, they conform to the intents, purposes and requirements of the Act or any existing law; and it bars challenges to service of notices, orders or communications where the recipient has acted upon them or failed to contest service in earlier related proceedings.