Withholding is your reponsibility. Even if a mistake was made on your W-4, it is not their fault that you owe taxes for the year. The only thing that has changed is that you have received your full paycheck, and now must pay it in one lump sum. But it does definitely create a problem to have to pay it all in one lump sum. I suggest putting enough aside from your next few paychecks, and pay on April 15th.
No, you are always responsible to pay your income taxes. What your employer has done is not legal and you should report these actions to the department of labor in your state. You should first ask your employer to correct there actions and treat you as an employee in the future and correct how they prepare your pay check.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.