A reader wrote in and asked, "What do I do if I discover that I goofed and submitted a tax return with an error in it? Turn myself in to the IRS?"
The answer is: Not exactly. The tax form you prepare and mail in isn't the only one you're ever allowed to file for that year. After you've filed the regular return, if you need to, you can file an amended return via Form 1040X. In fact, you can even amend an amended return.
You're permitted to amend your return until three years have passed since the date the original return was filed (or its due date). Also, if any of your changes affect your adjusted gross income, you'll probably want to amend your state return, too. Contact your state tax department for the appropriate forms.
For example, imagine that it's the year 2039 and the rules regarding amended return deadlines haven't changed. A woman named Hester files her return for the tax year 2038 on April 4, 2039 -- 11 days before the filing deadline of April 15, 2039. She'll have three years in which to amend her return. The last day on which she can file an amendment will be April 15, 2042, three years after the initial due date.