Have you actually e-filed your return?  Did you hit the Transmit/Submit button?  (or Did you Print and Mail your return?)

If so, You need to WAIT until you find out if your return has been REJECTED or ACCEPTED.

Then it is BEST if you wait until you actually receive the refund (or payment due is taken from your account) based on your already filed return;  then you can start the amendment process

If it has been rejected then you can simply make the corrections required to fix the rejection AND at the same time add the new information, and re-submit your CORRECTED e-filed return.

If it has been accepted you will have to prepare and mail an amended return ---BUT WAIT until you receive your refund before filing the amended return.

A Form 1040-X amended return MUST be PRINTED and MAILED; and usually takes 3-4 months to be processed.

Perhaps some terminology/definitions would be in order:

AMEND:- your return is what you do AFTER your return has been filed and accepted by the IRS and/or state.  (No additional charge)

CHANGE:- your return is what you do BEFORE your return has been filed --simply go back to the respective information and change it as needed.

CORRECT:- your return is what you do AFTER you attempted to file your return and it is rejected by either IRS, or state --again, simply go back to the respective information and change it as needed.

Above all  DO NOT DO ANYTHING until you find out if your return is rejected or accepted

NOTE:  When mailing a printed return it is always a good idea to go to the Post Office and mail it 'Certified' with 'Return Receipt'.  That way you will have proof that they received it and when it was received.

It has been filed and ACCEPTED. 

Let me repeat the situation again:

1. on 4/12, 1040 was filed and accepted by IRS; refund = $5000

2. on 4/14 I discovered additional income (1099INT) that I overlooked previously. The extra tax on this is $300.

QUESTIONS:

1. why wait for the refund before filing 1040X? Is there a high chance that my refund will actually be 5000-300=$4700, meaning they will automatically add the missing 1099INT?

2. If they don't do this automatically and send me $5000, won't I owe interest / penalty on $300?

2. maybe I should PAY $300 today (4/15) but wait to FILE the amended return until I get the refund? But what if they refund $5300 because they didn't match the extra income?

I FORGOT TO ENTER MY 1099-INT, WHAT SHOULD I DO NOW, I FILED MY RETURNS TODAY

Please see @bluedeb's post above. If your tax return is rejected then you'll be able to make corrections and resend it but if it is approved you'll need to amend your return.

How do I amend my 2019 TurboTax Online return? - Community

Edited 02/29/20

I have read through this whole thread because today I got my bank interest for $100.00 I did not report, because when I checked there was none.  I filed on 1/27 and today is 2/1.  It is currently accepted and in processing.  Being a 50/50 chance they don't catch it and I get my return as is, I will do an amendment. How do I figure out the taxes owed on it to send a check with it?  I had turbo tax do my taxes to figure these things out, I have no idea what to owe on it like the original question.

Can I do the amendment through turbo tax and it tell me?  I feel kind of stuck right now and wanna be prepared.  In all reality I need my return asap because I am broke and since it was accepted does that mean they will catch it?

Thanks!

You can do an amendment through TurboTax. The program will tell you how much you will owe when you amend your return.

If your return was accepted, the IRS won’t send it back so you will have to amend your tax return.

Adding $100 should not be much of an extra tax. The maximum tax rate is 37% or $37 dollars, but you’ll probably owe a lot less.

See How to Correct Federal Tax Returns

Thank you! I know a couple years ago I forgot the marketplace form and got a letter in the mail asking for it, it totally delayed the process and stayed in review for a bit longer than I liked.  However that was a lot more than $100,  I am assuming they will let it go and I file the amendment and pay the tax on $100 bucks.

Hello, when you replied that the IRS won't send it back, do you mean the tax return itself? I am in the same boat. IRS accepted my return, but my return has not been sent to me. Should I file the amendment now or wait until I receive the return and/or letter from IRS? I have not issue sending in the amended return now.

You should amend your return now to take care of the income that wasn't reported. 

The IRS will eventually contact you about the missing unreported income at some point in time. That could happen in a few weeks or even sometime this summer. But they will contact you if their records do not match what you reported.

@jsmc1