Determination of tax shortfall: notice-based assessment, interest and penalty regime with time-bar and pre-payment relief available. Determination of unpaid or short-paid tax, erroneous refunds, or wrongly availed input tax credit (other than due to fraud or wilful mis-statement) is effected by a show-cause notice requiring payment of tax with interest under section 50 and penalty; a supplementary statement may be deemed notice where the same grounds apply; pre-payment by the taxpayer can prevent issuance of notice or penalty for that amount, but the proper officer may issue notice for any shortfall and must issue final orders within the statutory limitation period.