You may have received a check in Jan for work performed in Dec. You may have also received a commission check later. The W-2 is determined by when you received the money, not when you earned it. Also, as far as amending the return, if it's a small amount then just let the IRS adjust your return. That costs you nothing except perhaps a few more dollars in taxes due. If they charge penalty and/or interest, then just write them a letter to request a waiver of the P&I due to the fact that you had already filed your return before receiving the W-2 which you were not expecting. They will usually waive it unless you are a habitual offender. Good luck.
