Determination of tax shortfall triggers notice, interest and penalty where tax is unpaid, short paid, refunded or wrong credit. If a proper officer finds tax unpaid, short paid, erroneously refunded, or input tax credit wrongly availed or utilized for reasons other than fraud or willful misstatement or suppression, the officer shall serve a show cause notice requiring payment of the specified tax with interest under section 50 and a penalty. A subsequent statement for additional periods with the same grounds is deemed a notice. Voluntary payment of tax and interest before issuance prevents notice and penalty; payment within thirty days of notice removes penalty. The officer must determine and order tax, interest and penalty within three years from the annual return due date or erroneous refund.