Yes, it is a mistake. And many taxpayers inadvertently do it.

Certain items of income and deductions, which are not in the form 16 become a part of the ITR, which may change your tax liability by a significant amount.

Now, to suggest you a remedy I strongly recommend you to check these below items:

1. Have a look at your saving bank account statements. Add all the bank interests you received. Many banks give interest in 6 months (Sep and Mar) and some pay every quarter.
2. Download form 26AS from traces portal. Check whether it shows any other income other than salary. Also check the TDS deducted on them. Add all the income. Add all the TDS deducted too. Generally you will find Fixed deposit interest here.
3. Match the investments disclosed in form 16 with your actual investments. Note down the variance.
4. Check if you can claim interest and principal deduction on repayment of loans (housing loans etc).

If you find any of the above, then please quickly revise your income tax return.

If you have only saving bank interest income that is less than Rs 10000 then you may not like to revise your return but I suggest you to revise in this case too.

PS: There are many instances, almost 3 in every 5 cases where the form 16 doesn't fully resemble the income tax return. I can tell you that because I regularly file Income tax returns for various persons.

PSS: You can revise your tax only if you filed your original tax return in time.

Thanks for reading.