If there is any underpayment of tax on your return due to fraud, a penalty of 75 percent of the underpayment due to fraud will be added to your tax. The fraud penalty on a joint return does not apply to a spouse unless some part of the underpayment is due to the fraud of that spouse.
Negligence or ignorance of the law does not constitute fraud.
Typically, IRS examiners who find strong evidence of fraud will refer the case to the Internal Revenue Service Criminal Investigation Division for possible criminal prosecution. Keep in mind that both civil sanctions and criminal prosecution may be imposed.