Eight common GSTR-1 and GSTR-3B filing mistakes Indian businesses still make in 2026 — and how to avoid notices, interest, and blocked input tax credit.
GSTR-1 and GSTR-3B together carry your entire monthly GST narrative — outward supplies, input tax credit claims, and tax payment. With auto-population of GSTR-3B from GSTR-1 and the new restricted-editing field becoming permanent in FY 2026-27, even small mistakes now trigger automated ASMT-10 notices, blocked ITC, and interest under Section 50. This guide flags the most common GSTR-1 and GSTR-3B mistakes Indian businesses still make — and how to avoid each one.
Mistake 1 — wrong GSTIN of the recipient
A single wrong GSTIN means the recipient cannot claim ITC, and you may have to issue a credit note plus a fresh invoice. Always validate the GSTIN on the GST portal or via your accounting software's auto-lookup before raising any B2B invoice.
Mistake 2 — supply classified as B2C instead of B2B
Reporting a B2B sale as B2C blocks ITC for the recipient. Use the customer's GSTIN diligently and configure your billing system to fail an invoice when the customer profile shows "Registered" but no GSTIN is entered.
Mistake 3 — wrong place of supply
Place of supply decides whether to charge CGST+SGST or IGST. Common errors include treating an inter-state stock transfer as a local sale or vice versa. Re-check Section 10 to 13 of the IGST Act for each transaction type.
Mistake 4 — incorrect HSN/SAC code
- Taxpayers with turnover above ₹5 crore — 6-digit HSN mandatory at line-item level.
- Taxpayers with turnover up to ₹5 crore — 4-digit HSN for B2B and optional for B2C.
- Services — 6-digit SAC code mandatory for B2B.
- HSN-rate mismatch triggers an immediate validation warning on the portal.
Mistake 5 — over-claiming ITC in GSTR-3B
ITC claim is now capped at the figure reflected in GSTR-2B. Claiming higher ITC triggers Rule 88C/88D notices (DRC-01C, DRC-01B) requiring a written explanation and possible reversal with interest. Always reconcile GSTR-2B before locking GSTR-3B.
Mistake 6 — missing reverse charge entries
Many taxpayers report inward supplies under RCM (e.g., legal services, GTA, import of services) but forget to pay tax in cash and then claim ITC on the same. RCM cannot be discharged through ITC — it must be paid through cash ledger and then claimed as ITC in the next month.
Mistake 7 — wrong reporting of credit and debit notes
- Credit notes must be reported in the month of issue, not the original invoice date.
- Time limit for issuing credit notes — 30 November following the financial year (per Finance Act 2022).
- Debit notes increase taxable value — ensure the period is within the same FY for ITC eligibility on the recipient side.
Mistake 8 — late filing and interest miscalculation
- Late fee — ₹50 per day for GSTR-1 and ₹50 per day for GSTR-3B (₹20 per day for NIL returns), capped per the latest CBIC notification.
- Interest at 18% per annum on net tax liability under Section 50(1).
- ITC blocking under Rule 37A if the supplier has not paid tax — reverse the ITC and reclaim later if needed.
Cross-statement reconciliation
The single biggest control against GST mistakes is a monthly five-point reconciliation. Compare your sales register with GSTR-1 (outward supplies), GSTR-1 with the auto-populated GSTR-3B (Table 3.1), GSTR-2B with your purchase register (ITC), RCM payable with cash ledger payment, and credit notes booked with credit notes reported. If these five tie out every month, your annual GSTR-9 and 9C reconciliation becomes a formality.
Annual return implications
Mistakes left uncorrected through the year flow into the annual GSTR-9 and 9C reconciliation, where every difference must be explained in writing. Department audits routinely start from GSTR-9 mismatches and ASMT-10 notices. The cheaper, faster path is to fix every monthly mismatch in the next return cycle rather than letting them aggregate.
ITC reversal under Rule 37
Rule 37 of the CGST Rules requires reversal of input tax credit if payment to the supplier is not made within 180 days from the date of invoice. This is one of the most overlooked GST controls. Build an aged-payables report at the end of each month, flag invoices crossing 150 days, and either pay before 180 days or reverse the ITC with applicable interest. The reclaim is allowed once payment is actually made — but the interest cost in between is real.
Conclusion
GSTR-1 and GSTR-3B errors compound month after month. Run a five-point reconciliation — sales register vs GSTR-1, GSTR-1 vs GSTR-3B, GSTR-2B vs purchase register, RCM vs cash payment, and credit notes vs original invoices. For FY 2026-27, treat this checklist as non-negotiable before clicking "File".
