Validity of proceedings: procedural defects do not invalidate assessments or notices if substance conforms to the Act. The provision protects assessment, re-assessment, adjudication, review, revision, appeal, rectification, notices, summons and related proceedings from being invalidated solely for mistakes, defects or omissions when such proceedings, in substance and effect, conform to the intents and requirements of the Act or any existing law; it further prevents challenge to service of notices, orders or communications where the addressee has acted on them or where service was not previously questioned in earlier related proceedings.