Procedural defects in tax proceedings do not invalidate assessments if their substance conforms to statutory requirements and prior actions stand. Assessment and related proceedings will not be invalidated for 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. Similarly, service of a notice, order or communication cannot be challenged where the recipient has acted on it or failed to contest its service in earlier proceedings arising from that communication.