Curative GST procedure provision preserves assessment and notice proceedings despite technical defects when substance matches statutory requirements. Assessment, reassessment, adjudication, review, revision, appeal, rectification, notice, summons and other proceedings under the GST law are not invalidated merely because of a mistake, defect or omission, where the proceeding is in substance and effect consistent with the intents, purposes and requirements of the Act or existing law. The provision also protects service of any notice, order or communication once it has been acted upon by the recipient, or where service was not challenged at the earlier stage.