Taxpayers should file amended returns when a return that has already been filed is incomplete or in error. Use Form 1040X for amended returns.
Even when the original return has an error, an amended return is not always required. For example, the IRS will usually correct a math error on a return, or they will write the taxpayer and request a missing schedule or form. In these cases, when the taxpayer has received a notice from the Internal Revenue Service, do not amend the return; instead, refer the taxpayer to the contact person and telephone number on the notice.
Taxpayers in a legal same-sex marriage may choose to file original or amended returns if they want to be treated as married for one or more prior tax years still open under the statute of limitations.
Additionally, if an employer provided health coverage for an employee's same-sex spouse, the employee may claim a refund of income taxes paid on the value of coverage that would have been excluded from income had the employee's spouse been recognized as the employee's legal spouse for tax purposes.
Math errors are more frequent with paper returns than e-file returns.