Most audits are about money — whether a deduction holds up, whether income was reported correctly. A small number are about something far more serious: whether the taxpayer intended to cheat. The civil fraud penalty under § 6663 is the IRS’s most severe civil sanction, and the examination in which it surfaces — the “eggshell” audit — is the most dangerous kind a taxpayer can face, because the same proceeding can quietly turn into a criminal investigation. Understanding both the penalty and the dynamics of the audit is essential to navigating either.

The civil fraud penalty — § 6663

Section 6663 imposes a penalty equal to 75% of the portion of an underpayment attributable to fraud. It is reserved for intentional wrongdoing — a deliberate attempt to evade a tax known to be owing — and it applies only where a return was filed (a fraudulent failure to file is addressed by § 6651(f), which raises that penalty to 15% per month, up to 75%). The accuracy-related penalty and the fraud penalty cannot both be charged on the same portion of the same underpayment; the IRS must choose, though it often pleads fraud with accuracy as an alternative.

The burden runs the other way

Unlike almost everything else in a tax dispute, the burden on fraud is the government’s. The IRS must prove fraud by clear and convincing evidence — a markedly higher standard than the usual preponderance — under § 7454(a) and Tax Court Rule 142(b). It must show the taxpayer intended to conceal, mislead, or otherwise defeat the tax. But there is a sting in the tail: once the IRS establishes that any portion of the underpayment is fraudulent, § 6663(b) treats the entire underpayment as fraudulent unless the taxpayer proves — by a preponderance — that some part was not. And because fraud is rarely admitted, the IRS proves it circumstantially, through the “badges of fraud.”

Badge of fraudWhat it looks like
Understating incomeOmitted receipts, unreported cash
Inadequate or missing recordsNo books, or records destroyed or altered
Concealment of assets or incomeNominees, hidden or offshore accounts
Dealing in cashLarge cash transactions to avoid a paper trail
False statements or documentsLying to the agent; fabricated paperwork
Implausible or inconsistent explanationsConduct inconsistent with honest error
Failure to cooperateObstruction or evasiveness during the exam
Illegal-source incomeProceeds of unlawful activity
The badges, not a confessionNo one writes down an intent to evade. Courts infer it from patterns — understated income, missing records, cash dealing, false statements, concealment. No single badge is decisive; the IRS assembles them into a clear-and-convincing whole, and the defense is to supply the innocent explanation the badges leave out.

The eggshell audit

An eggshell audit is a civil examination of a return that contains real exposure — materially understated income or overstated deductions — where the cause may not be mere negligence but willful conduct. The name captures the danger: beneath an ordinary-looking civil audit lies a fragile shell of potential fraud, and a careless step can crack it. The same set of facts can resolve as a 20% accuracy penalty or escalate into a criminal referral and a 75% fraud penalty; intent is the hinge.

CIVIL EXAMINATIONa return is under auditNEGLIGENCE OR GOOD-FAITH ERRORan honest mistake, not intent§ 6662 ACCURACY PENALTY — 20%civil, and that is the end of itFIRM INDICATIONS OF FRAUDintent to evade appearsEXAM SUSPENDED → REFERRED TO CIthe agent does not tell youCRIMINAL CASE + § 6663 FRAUD — 75%prosecution and the 75% penaltyReverse eggshell audit: a criminal investigation disguised as a civil exam.The peril is what you say and produce during it — new false statements are themselves crimes.
The same audit can branch two ways. The difference between a 20% penalty and a criminal referral is intent — and the agent who sees it coming is not required to warn you.

What makes it so dangerous

The peril is structural. A revenue agent who develops “firm indications of fraud” is instructed to suspend the civil examination and refer the matter to Criminal Investigation — without telling the taxpayer. From that point the civil audit may be a criminal investigation in disguise (the “reverse eggshell” audit, where parallel civil and criminal tracks run at once). Meanwhile, everything the taxpayer says and produces is evidence, and a fresh false statement to the agent is itself a new crime under § 7206 or 18 U.S.C. § 1001. The instinct to “explain it away” in a friendly conversation with the examiner is exactly the instinct that does the most damage.

The agent will not warn youWhen an examiner sees firm indications of fraud, the rules tell them to stop and refer the case quietly, not to caution the taxpayer. By the time a referral is obvious, the record is often already made. In any audit with real exposure, the safe assumption is that intent is on the table.

Managing the exposure

The defense to an eggshell audit is discipline, not improvisation. Counsel should control the flow of information, assert the attorney-client privilege and, where the accountant’s work is involved, a properly structured Kovel engagement; preserve the Fifth Amendment where testimony would be incriminating; and never let the client supply a new false statement to paper over an old one. Where the conduct was willful, the question of a voluntary disclosure — made before any referral — has to be evaluated early, because that window closes the moment the IRS is already looking. The interaction with the summons power and the § 7602(d) referral bar is covered in our summons post.

The practical takeaway

Civil fraud is the rare tax matter where the government carries the heavy burden — clear and convincing evidence of intent — which is precisely why the badges of fraud and the conduct of the examination matter so much. The single most important rule in an eggshell audit is to recognize it for what it is before talking: an exam where the stakes are not a percentage of the tax but potentially prosecution. That recognition, early, is the whole game.