There is no specific penalty for the late filing of Form W-9. Form W-9 is typically used to provide taxpayer identification information to entities that are required to report certain payments to the Internal Revenue Service (IRS), such as income earned as an independent contractor. Failure to provide a valid Form W-9 may result in backup withholding on payments made to the taxpayer, which is a withholding of a flat percentage (currently 24%) of the payment to ensure tax compliance. So, the penalty for not filing a Form W-9 would be the backup withholding of a portion of the payment.