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W-4 was not updated when moving from part-time to full-time, still showed Tax-Exempt. What happens now?
The W4 specifies withholding for income taxes, FICA taxes are not impacted. The tax withholding is do that you do not need to make estimated tax payments. Failing to make sufficient quarterly estimated tax payments or withholding a sufficient amount could result in you being hit with under payment penalties but nothing more. The under payment penalties will be figured out as part of you income tax return.  What you should have done when you discovered this was use the extra withholding line on the W4 to further increase your withholding. The nice thing about withholding is that you back load it and the IRS does not care.  The company has no liability here. It is your responsibility to update them when your personal circumstances change. You will be fully responsible for the tax bill.  There is no company paid portion of your income tax so they are not impacted. The company only pays an employer share of FICA and that is not impacted by how you fill out the W4. First thing to do is figure out how much you owe the IRS. Then determine if you can pay it or if you need to investigate an installment option. In any case make sure to file your return on time.