If you got another form like a W-2 after filing or you forgot to add something you can still fix your return.
You'll want to determine if the IRS or state has accepted the information you already filed. If the IRS or state has accepted it, you'll see Accepted as the status of your return when you sign in to TurboTax (in Tax Home) or when you open the CD/Download Turbotax software. If they have accepted your return, then you may need to amend your tax return.
If your return was rejected, you can add the missing form or information, correct the reason for rejection and resubmit.
If you forgot to add something like dependent information, a deduction you qualify for, or even used the wrong filing status it can have a significant impact to your tax bottom line (your refund or amount due). Some info doesn't make a difference monetarily, but can still be something the IRS or state needs corrected. Your best bet is to correct (amend) the information and send it back to the IRS or state.