Substance over form: procedural defects in tax proceedings do not invalidate actions if they conform to statutory intents. Section 160 preserves the legal validity of assessment, re-assessment, adjudication, review, revision, appeal, rectification, notices, summons and related proceedings despite mistakes, defects or omissions, where such proceedings are in substance and effect in conformity with the intents, purposes and requirements of the Act or any existing law. It also bars questioning service of a notice, order or communication if the recipient has acted upon it or failed to challenge service in earlier related proceedings.