Can an employer take out taxes if a Form W-4 was never filed?

Yes, the employer is required to withhold income taxes. Publication 15, Circular E, Employer's Tax Guide, states that if an employee does not give you a completed Form W-4 (PDF), Employee's Withholding Allowance Certificate, withhold tax as if he or she is single, with no withholding allowances.

The employer is also required to withhold social security and Medicare taxes.

If an employee claims more than 10 exemptions on their Form W-4, does the employer have to report this to the IRS?

Yes, if you receive a Form W-4 (PDF), Employee's Withholding Allowance Certificate, on which the employee claims more than 10 withholding allowances, you must send a copy of that Form W-4 to the IRS service center with your next Form 941, Employer's Quarterly Federal Tax Return employment tax return.

Also, if an employee claims exemption from withholding and his or her wages would normally be expected to exceed $200 or more a week, you must also send a copy of that Form W-4 to the service center with your next employment tax return.

If you want to submit the Form W-4 earlier, you can send a copy of the Form W-4 to the IRS with a cover letter, including your name, address, employer identification number, and the number of forms included. The service center will send you further instructions if it determines that you should not honor the Form W-4. Follow the W-4 unless you hear back from the IRS.