What to Do If You Realize You Made a Mistake After Filing Your Taxes
First, take a deep breath. Mistakes happen. And this one is a pretty easy fix. When you make a mistake on your tax return either because you left off some information or failed to file a particular schedule, you simply correct your mistake by filing a federal form 1040X, Amended U.S. Individual Income Tax Return.Filing an amended return is fairly straightforward. To fill out the form, you'll want to have your previously-filed return handy. The federal form 1040X requires you to list the originally-reported information, the change and the correct amount at Part I.
If you're making a change to your exemptions, you'll also complete Part II; the same process of listing the originally-reported information, the change and the correct amount applies. At Part III, the IRS asks for a quick explanation of why you're filing an amended return. You don't have to write a novel here -- just jot down a few sentences to explain what happened. This could be as simple as "received another W-2 in the mail after I filed."
If you need to correct more than one tax return, file a separate federal form 1040X for each year you are amending. Also keep in mind that if you're filing an amended federal return, you may also need to file an amended state return.
Of course, an amended return isn't used for every situation. If you need to correct your address (hey, it happens), for example, don't use the form 1040X; instead, use a federal form 8822, Change of Address. You also don't want to file an amended return to request an abatement of penalty or a refund of penalties and interest -- use a federal form 843, Claim for Refund and Request for Abatement instead. Finally, don't file an amended return for an injured spouse claim; you'll want to file federal form 8379, Injured Spouse Allocation for that.
Do not simply file another federal form 1040. The IRS will not understand that the second return is a corrected return and will kick it back. Filing a second return rather than an amended return may also delay your refund.
Expect to wait two to three months for your amended return to be processed. If your amended return results in a refund, be patient since it won't be processed as quickly as an original return. If you owe additional tax, you may owe interest and possibly penalties, depending on your situation, so be sure to send in a payment with your amended return if applicable; you'll also want to file that amended return as soon as possible to keep interest and potential penalties down.
Your best bet is to be as careful as possible when filing your original return in order to reduce the possibility of making mistakes. A good rule of thumb is to use last year's tax return as a guide to make sure you've accounted for your income and deductions. If you're filing with a software package, run an error check (most tax software packages have one), and if you're doing the return by hand, go over your figures with a calculator. If you still make a mistake, remember that filing an amended return can be relatively quick and painless.