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.
Math errors are more frequent with paper returns than e-file returns.