Mistaken GST classification: refund is required under Section 77 rather than adjusting IGST with CGST/SGST. A taxable person who has paid tax under the wrong GST head because of an incorrect interstate/intrastate classification cannot adjust CGST/SGST against wrongly paid IGST or vice versa; the appropriate remedy is a refund of the tax so paid under the statutory refund mechanism set out in the GST law, specifically Sec.77.