What makes a return defective, and the 15-day clock
Where the assessing officer considers a return defective, Section 139(9) requires the department to intimate the defect and give you 15 days to rectify it, extendable if you apply. The consequence of not curing it in time is the sharp part: the return is treated as an invalid return, meaning it is as if you never filed, so the filing and any refund fall away and you are exposed to the non-filer consequences that follow from that.
There is a small mercy: if you cure the defect after the 15 days but before the assessment is actually made, the officer may condone the delay and treat the return as valid. But that is discretionary, so the safe course is always to fix it inside the window rather than rely on condonation.
The defects that catch NRIs
For non-residents the defects are usually structural rather than about the numbers. The most common is the wrong ITR form: a non-resident cannot use ITR-1, so a return filed on it is defective and has to be redone on ITR-2 or ITR-3 depending on your income. Another frequent one is a missing schedule, the residential-status details, the capital-gains schedule, or a foreign-asset schedule where it applies, left blank or omitted.
The third common trigger is a mismatch between the tax and TDS claimed and what the department has on record, where the return claims a credit that does not reconcile with your Form 26AS. Each of these is a fixable defect, but each requires the return to be corrected and refiled correctly, not just an explanation, which is why the right ITR form and complete schedules matter from the start.