Validation of tax proceedings: substantive conformity prevents invalidation for procedural mistakes, and notice challenges limited where acted upon. Assessment, re-assessment, adjudication, review, revision, appeal, rectification, notices, summons and related proceedings under the Act are not invalid merely due to mistakes, defects or omissions if they are in substance and effect in conformity with the intents, purposes and requirements of the Act or any existing law; and service of notices, orders or communications cannot be challenged where the recipient has acted on them or failed to question their service in earlier related proceedings.