There are serious penalties that could be assessed if the IRS feels that you willfully chose not to report your income on your tax return. You are required to report your income and expenses on your tax return, if you were required to file a tax return for the year. You can amend your return, and include the additional W-2. You will need
to wait for the original tax return to be accepted before
you amend your return. Please keep in mind that an amended return, Form
1040X, can only be printed and mailed to the IRS, it cannot be e-filed, but you
can prepare your 1040X in TurboTax and print it for filing.
Please see the following TurboTax FAQ for instructions
on how to amend your 2015 tax return in TurboTax.
https://ttlc.intuit.com/replies/3288565
You can read more at the following IRS link: https://www.irs.gov/publications/p17/ch01.html#en_US_2015_publink1000170711
If you don't file your return and pay your tax by the due date, you may have to pay a penalty. You may also have to pay a penalty if you substantially understate your tax, understate a reportable transaction, file an erroneous claim for refund or credit, file a frivolous tax submission, or fail to supply your SSN or individual taxpayer identification number. If you provide fraudulent information on your return, you may have to pay a civil fraud penalty.
Filing late. If you don't file your return by the due date (including extensions), you may have to pay a failure-to-file penalty. The penalty is usually 5% for each month or part of a month that a return is late, but not more than 25%. The penalty is based on the tax not paid by the due date (without regard to extensions).
Fraud. If your failure to file is due to fraud, the penalty is 15% for each month or part of a month that your return is late, up to a maximum of 75%.
Paying tax late. You will have to pay a failure-to-pay penalty of ½ of 1% (0.50%) of your unpaid taxes for each month, or part of a month, after the due date that the tax isn't paid. This penalty doesn't apply during the automatic 6-month extension of time to file period if you paid at least 90% of your actual tax liability on or before the due date of your return and pay the balance when you file the return.
The monthly rate of the failure-to-pay penalty is half the usual rate (0.25% instead of 0.50%) if an installment agreement is in effect for that month. You must have filed your return by the due date (including extensions) to qualify for this reduced penalty.
If a notice of intent to levy is issued, the rate will increase to 1% at the start of the first month beginning at least 10 days after the day that the notice is issued. If a notice and demand for immediate payment is issued, the rate will increase to 1% at the start of the first month beginning after the day that the notice and demand is issued.
This penalty cannot be more than 25% of your unpaid tax. You won't have to pay the penalty if you can show that you had a good reason for not paying your tax on time.
Combined penalties. If both the failure-to-file penalty and the failure-to-pay penalty (discussed earlier) apply in any month, the 5% (or 15%) failure-to-file penalty is reduced by the failure-to-pay penalty. However, if you file your return more than 60 days after the due date or extended due date, the minimum penalty is the smaller of $135 (adjusted for inflation) or 100% of the unpaid tax.
Accuracy-related penalty. You may have to pay an accuracy-related penalty if you underpay your tax because:
You show negligence or disregard of the rules or regulations,
You substantially understate your income tax,
You claim tax benefits for a transaction that lacks economic substance, or
You fail to disclose a foreign financial asset.
The penalty is 40% of any portion of the underpayment that is attributable to an undisclosed noneconomic substance transaction or an undisclosed foreign financial asset transaction. The penalty won't be figured on any part of an underpayment on which the fraud penalty (discussed later) is charged.
