A few causes for a defective return

If a person is showing a lesser income in his tax return than the income appearing in the form 26AS of that Financial Year then the return will be processed as defective.
If the tax return is filed with showing the higher TDS than the TDS appearing in form 26AS then the return will be processed as defective.
If personal particulars like DOB, father name etc is entered that are mis-matching with the PAN database then income tax return will be declared as defective.
If a person is opting presumptive method to file the ITR and he misses to declare to the financial particulars then filing of return can be treated as defective return.
If a person is opting presumptive method to file the ITR and shows his profit under 8% or 6% and in case of section 44AD below 50%, as the case may be, then his return will be treated as defective return.
If turnover of a person is exceeding the limit of tax audit requirement and he files his income tax return without opting for tax audit then his return will be declared as defective return.
At the time of filing the Business Income Tax Return, if a person is claiming the benefit of depreciation but the chart of deprecation is not filled and/or not correctly filled then the return may be treated as defective.
While uploading the tax return, the option of tax audit is selected YES but mandatory fields of profit and loss and balance sheet are not filled then income tax return will be declared defective.