Validity of proceedings preserved when procedural mistakes do not defeat substantive conformity with statutory requirements. Proceedings under the GST framework - including assessment, reassessment, adjudication, review, revision, appeal, rectification, notices and summonses - shall not be invalidated solely for a mistake, defect or omission if in substance and effect they conform to the intents, purposes and requirements of the Act or earlier law; service of notices, orders or communications is not to be questioned where the recipient has acted upon them or failed to raise the issue at the earliest available stage.