What happens if your employer doesn't cut federal tax from your check?

Presuming you are an employee, paid on the books...he abolsutely must, or report him to your local department of labor. That is because, he is required to pay things, like half of your FICA, Unemployment Insurance, Worker Comp, etc on your behalf, and not charge you. As to the income tax withholding....in a way that is the least important. His not withholding doesn't change your obligation to report and file, or the maount you would ultimately pay. Just that you should have made stimated payments in through the year (or pay a penalty for not doing so). But, in virtually all other ways, you come out the same...you will pay the IRS the amount of tax you owe.