It depends on the type of mistake that was made. Many mathematical errors are caught in the processing of the tax return itself. If you did not attach a required schedule the service will contact you and ask for the missing information.

If you did not report all your income or did not claim a credit, you are entitled to file an amended or corrected return using Form 1040X (PDF), Amended U.S. Individual Income Tax Return. Include copies of any schedules that have been changed or any Form W-2 (PDF) you did not include. The Form 1040X (PDF) should be submitted after you receive your refund or by the due date of the return, whichever, is earlier. Generally, to claim a refund, the Form 1040X (PDF) must be received within three years after the date you filed your original return or within two years after the date you paid the tax, whichever is later.

References:

• Form 1040X (PDF), Amended U.S. Individual Income Tax Return
• Tax Topic 308, Amended returns

Source: www.irs.gov