Validity of proceedings: procedural defects do not invalidate actions if they substantially conform to the Act and are acted upon. Section 160 provides that assessments, adjudications, reviews, revisions, appeals, rectifications, notices, summonses and other proceedings under the Act are not invalidated by 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 that service of notices or orders cannot be questioned where the recipient has acted on them or failed to challenge the service in earlier related proceedings.