Validity of tax proceedings despite procedural defects upheld when actions conform to legislative intent. Proceedings under the Act shall not be invalidated solely for mistakes, defects or omissions if, in substance and effect, they conform to the intents, purposes and requirements of the Act or earlier law; this covers assessments, adjudications, appeals, rectifications, notices and related proceedings. Service of a notice, order or communication cannot be questioned where the addressee has acted on it or failed to challenge service at the earliest proceedings commenced, continued or finalised pursuant to that communication.