If you find an error after filing your return, you may need to amend your return. The IRS may correct errors in math on a return and may accept returns without certain forms or schedules. In these instances, there is no need to amend your return! However, do file an amended return if there is a change in your filing status, income, deductions, or credits.
Generally, to claim a refund, you must file Form 1040X within 3 years after the date you filed your original return or within 2 years after the date you paid the tax, whichever is later. Returns filed before the due date (without regard to extensions) are considered filed on the due date.