GSTR-1 is your outward supplies, GSTR-3B is your summary return. The two must agree — and increasingly, the GSTN flags differences automatically. Mismatches often arise from unreported credit notes, wrong place-of-supply, or B2C-vs-B2B classification errors. Build a 15-minute monthly reconciliation into your closing checklist; it pays for itself the first time it prevents a Section 75 notice.