Validity of tax proceedings preserved despite clerical defects if substance complies with law; service not challengeable once acted upon. Proceedings under the Act, including assessments and related processes, are not invalid merely for mistakes, defects or omissions if in substance they conform to the intents, purposes and requirements of the Act or any existing law. Service of a notice, order or communication shall not be called into question where the recipient has acted on it or where its service was not previously contested in proceedings arising from it.