I didn't file two of my W2 forms because I thought I only had one. What happens if I don't file them this year?

If your tax return has been filed, be sure to wait until it's accepted by the IRS before amending it. You should wait because on the small chance that your return is rejected, you won't have to amend it. You can just add your other W-2s while you're fixing whatever caused the reject. If the return has been accepted, then you will need to amend it to add all W-2s you received for work you did in 2014.

You need to file an amended return for the year you failed to include the two W-2s. You need to put all the W-2s in for this year.