If you're in a lower income bracket, there could be no taxes due. Double check and make sure that you immediately change withholding status in the event that you believe you'll owe the IRS money for your projected income.
It's your responsibility to compare paystubs every time you get a check, and make sure that proper taxes are being withheld. Typing errors and getting in a hurry and omitting something could be the problem also, so make sure your W-2 is correct. Hope you don't end up oweing the IRS lots of money!