Substance-over-form principle preserves validity of GST proceedings despite procedural defects when they conform to law. Section 128 preserves the validity of assessment, adjudication and related tax proceedings notwithstanding mistakes, defects or omissions, where such proceedings are in substance and effect in conformity with the intents, purposes and requirements of the Act or earlier law; it also prevents challenges to service of notices or orders if the recipient has acted on them or did not raise objection at the earliest proceedings arising from those instruments.