GST errors rarely feel urgent when you make them. They become urgent later, when a customer calls because they cannot claim input tax credit on your invoice, or when your CA finds a mismatch a week before filing. Most of these errors are small, repeated, and completely avoidable.
1. Wrong or missing GSTIN
A single wrong character in the customer GSTIN means the invoice will not match in their books. They cannot claim credit, and you will be issuing a revised invoice. Always capture the GSTIN once, at the time you create the customer, and reuse it. Typing it fresh on every invoice is how mistakes happen.
2. Getting inter-state versus intra-state wrong
This is the most common structural error. If the place of supply is in your own state, the tax splits into CGST and SGST. If it is another state, it is IGST. Charging CGST and SGST on an inter-state supply is not a rounding issue, it is the wrong tax, and it has to be corrected.
3. Missing place of supply
For services especially, the place of supply decides everything about how the invoice is taxed. Leaving it blank because "the customer knows where they are" causes problems at filing time.
4. Invoice numbers that are not continuous
GST expects a continuous serial. Gaps, duplicates, or restarting the series mid-year create questions you do not want to answer during a review. Skipping a number because an invoice was cancelled is fine. Deleting the record entirely is not.
5. Wrong HSN or SAC code
Service businesses use SAC codes and often pick one at random the first time, then reuse it forever. It is worth confirming the correct code once with your CA and storing it against each service you sell.
6. Rounding inconsistently
If you round the tax on each line but your customer rounds the invoice total, the two sets of books will disagree by a rupee or two, enough to flag a mismatch. Pick one method and apply it everywhere.
7. No record of what was cancelled
Cancelled invoices still need to exist in your records as cancelled. Deleting them outright breaks the serial and removes the audit trail.
The pattern behind all seven
Every one of these errors comes from re-entering information that should have been stored once. Customer GSTIN, state, SAC code and tax rate: capture each of them a single time against the customer or the service, and the invoice builds itself correctly every time after that.
This article is general guidance, not tax advice. Confirm specifics with your chartered accountant.
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