Key takeaways
What this article covers, in order:
- Mistakes in what you claim
- Mistakes in what you report
- Mistakes in what you pay
- Mistakes in staying on top of it
- Quick self-audit
- How HelloBooks helps
Most GST trouble doesn't start with fraud or a big decision. It starts with something small: a hotel bill's GST claimed as credit, a lawyer's invoice where nobody paid reverse charge, a GSTR-1 that says one number while GSTR-3B says another. Those small things add up, and eventually a system-generated intimation arrives. Below are the twelve mistakes we see most often in small businesses, why each one hurts, and what to do instead.
Mistakes in what you claim
1. Claiming ITC on things the law blocks
Ashok runs a printing press in Coimbatore. He'd been claiming GST credit on the team's lunch bills, the owner's car insurance and Diwali gifts for clients. All three are blocked credits under Section 17(5). Food and beverages, motor vehicles for personal use (with some exceptions), gifts and personal consumption are the usual suspects.
The fix is to tag these expenses as "no ITC" when you record them, so the credit never reaches your GSTR-3B in the first place. Reversing later means interest.
2. Claiming credit that isn't in GSTR-2B
Your supplier gave you a proper tax invoice. You paid. But they haven't filed their GSTR-1, so the bill isn't in your GSTR-2B. Under the current rules, credit that doesn't appear in 2B isn't available yet. Claim it anyway and the system flags the excess, then sends an intimation.
Wait for it to appear, chase the supplier, and claim it in the month it shows up. For FY 2026-27 bills, you still have until 30 Nov 2027 to catch up.
3. Forgetting the 180-day payment rule
If you claim ITC and don't pay the supplier within 180 days of the invoice date, you have to reverse that credit with interest. You get it back once you pay. Businesses with long credit cycles, or disputes with a supplier, trip on this a lot. Check your payables ageing once a quarter.
Mistakes in what you report
4. GSTR-1 and GSTR-3B don't match
GSTR-1 says your output tax was ₹3,40,000. GSTR-3B paid ₹3,10,000. The ₹30,000 gap is exactly what the system looks for, and these days the 3B liability table is auto-filled from GSTR-1 for a reason. Common causes are an invoice added to GSTR-1 after 3B was filed, or 3B filed from a spreadsheet instead of the books.
File GSTR-1 first, check the auto-filled 3B against it, and only then pay.
5. Wrong place of supply
Charging CGST and SGST on what was actually an inter-state supply, or the other way round. The tax lands in the wrong government's account. Fixing it means a credit note, a fresh invoice under the right head, and a refund claim for the wrong tax. The mistake usually comes from a customer master with the wrong state, or from not asking where the goods are being delivered.
6. B2B invoices reported as B2C
If you don't capture the buyer's GSTIN, their invoice goes into the B2C summary and they lose the credit. Expect an annoyed phone call on the 15th, when they see their 2B. Make GSTIN a required field for business customers.
7. Wrong or missing HSN codes
Rate changes in Sep 2025 moved many items to new slabs. If your item master still carries old rates or vague HSN codes, every invoice inherits the error. Review your item list against current rates once, properly. It's a boring afternoon that saves a lot of correction later.
Mistakes in what you pay
8. Missing reverse charge
Some purchases make you, the buyer, liable to pay GST instead of the supplier. Legal services from an advocate, goods transport agency services in many cases, and certain other notified services are common examples, and renting commercial property from an unregistered landlord has also been under reverse charge for many businesses since Oct 2024. Pay the RCM tax in cash through GSTR-3B, then claim it as ITC if eligible.
Nobody remembers RCM until an auditor asks. Set a flag on the vendor so the system reminds you.
9. Treating interest as optional
If you pay tax late in cash, interest at 18% a year under Section 50 applies, whether or not anyone sends you a demand. Ignoring it doesn't make it go away; it shows up in a later notice with more interest. Pay it with the return.
Mistakes in staying on top of it
10. Not filing nil returns
"There were no sales this month, so I didn't file." Nil returns are still returns. Miss them and you collect late fees, your next return is blocked, and if you don't file GSTR-3B for two consecutive periods, your e-way bill generation can be blocked too. Keep missing them and registration can be cancelled.
11. Ignoring portal intimations
Mismatch intimations (like the ones for GSTR-1 vs 3B gaps, or excess ITC compared with 2B) give you a window to pay or explain. If you don't respond, the system can restrict your GSTR-1 filing for the next period. These land in your portal dashboard and email, and both are easy to miss. Log in to the portal at least once a month, even if your CA files for you.
12. Letting registration details go stale
You moved godowns, added a new branch, changed the bank account, or a partner left. None of it updated on the portal. Officers visit the registered address, notices go to old emails, and e-way bills go out with the wrong origin. Amend your registration within the time allowed whenever something changes.
Quick self-audit
| Check | How often | Mistake it catches |
|---|---|---|
| Expense ledgers tagged "no ITC" where blocked | When recording | 1 |
| Books ITC vs GSTR-2B | Monthly | 2 |
| Payables ageing over 180 days | Quarterly | 3 |
| GSTR-1 tax vs GSTR-3B tax | Before every 3B | 4 |
| Customer master: state and GSTIN | When adding customers | 5, 6 |
| Item master: HSN and rates | Once a year, and after rate changes | 7 |
| Vendors flagged for RCM | When adding vendors | 8 |
| Interest paid on late cash payment | With every late return | 9 |
| Return status for every period | Monthly | 10 |
| Portal notices and intimations | Monthly | 11 |
| Registration details vs reality | Whenever something changes | 12 |
If you find a mistake going back more than a year, or one involving large amounts, talk to a CA before correcting it. There's often a right order to fix things in.
How HelloBooks helps
Several of these mistakes come from records and returns living in different places. With HelloBooks Free you raise invoices with the customer's GSTIN, state and HSN codes, so place of supply and B2B reporting come out right, and you file GSTR-1 and GSTR-3B for one GSTIN directly to the GST portal from the same books. GSTR-2B reconciliation shows which bills you can actually claim, and AP ageing makes the 180-day check quick. Before you file GSTR-3B, HelloBooks shows the interest and late fee as the GST portal calculates them, so you're not guessing. It's ₹0 with no card and no expiry; see the free plan, HSN/SAC codes and GSTR-3B filing.
FAQs
What is the most common reason small businesses get GST notices?
Mismatches between returns: GSTR-1 vs GSTR-3B output tax, and ITC claimed vs GSTR-2B. Both are system-detected.
Can I claim GST on a business lunch or a client gift?
Generally no. Food and beverages and gifts are among the blocked credits under Section 17(5), with narrow exceptions.
What happens if I don't pay a supplier within 180 days?
You reverse the ITC you claimed on that bill, with interest, and can reclaim it once you pay.
Does not filing GSTR-3B affect e-way bills?
Yes. If GSTR-3B isn't filed for two consecutive tax periods, e-way bill generation can be blocked for your GSTIN.
I found an old mistake. Should I just fix it in this month's return?
Not always. Depending on the period and type, some corrections have time limits or need a DRC-03 payment. Check with a CA before you correct anything old.
None of these mistakes needs a genius to avoid. Just a monthly routine that you actually stick to.
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