What should you do when you discover an error on an already-filed tax return? In most cases, the answer is: prepare and file a correction or an amended return. Forms for amending already-filed returns typically have an “X” in the title. For example, you’d use Form 1040X to correct your individual federal income tax return, even if you originally filed Form 1040A or 1040EZ. Most payroll returns also have an “X” in the title to differentiate forms used for amendments (Form 941-X, for example), as do C corporations (Form 1120X). S corporation returns and information reports such as Form 1099 generally have a checkbox on the regular form to indicate you’re filing a corrected return.