You may also have to pay estimated tax if the amount of income tax being withheld from your salary, pension, or other income is not enough.

FORBES: Taxes From A To Z (2013): A Is For Annualized Income Installment Method

Not incidentally, that's also the reason "No Budget, No Pay" does not violate the 27th Amendment, which forbids salary increases or reductions during a current Congress: Their salary is not being reduced, it is being withheld until they actually do their job.

CNN: 'No Budget, No Pay' is no-brainer