Blog · 14 October 2026

Civil Fraud Penalty and the Statute of Limitations After Mitchell: §6663, §6501 and What US CPA Firms Should Tell Clients

By Keshav Bansal, CA

The civil fraud penalty under IRC §6663 is the IRS’s heaviest civil tool: 75% of the part of an underpayment that is due to fraud, and, through §6501(c)(1), the ability to assess at any time instead of within the usual three years. In Mitchell v. Commissioner, T.C. Memo. 2026-102 (filed September 30, 2026; Judge Kathleen Kerrigan), the Tax Court held that the IRS had not proved fraud on an attorney couple’s 2015 joint return. Without fraud, the statute of limitations had already run, so the $526,208 deficiency and the $394,656 §6663 penalty the IRS had determined both fell away. For US CPA firms, Mitchell is a clear reminder that a bad deduction is not the same thing as fraud, and that the assessment clock usually decides these cases.

What happened in Mitchell

Per the opinion (docket 9804-23, as published on Leagle), the IRS determined a 2015 deficiency of $526,208 and a $394,656 penalty under §6663 against Jesse Mitchell III and Darrilyn W. Mitchell. The issues were whether an exception to the limitation on assessment under §6501(c) applied, and, if so, the underlying deficiency and penalty.

Per Bloomberg Tax (October 5, 2026) and VitalLaw (October 1, 2026) reporting, the return reflected an advisor-recommended tax strategy built around a marketing company, marketing and management fees, and deductions for meetings held at home. Some of those deductions were improper or unsubstantiated under §162. VitalLaw’s Federal Tax Day summary (October 1, 2026) describes the disputed items as a loan, settlement fees and business expenses, and reports three key points:

• The IRS failed to establish the usual badges of fraud, including concealment, false records, implausible explanations, or a pattern of substantial understatement.
• The notice of deficiency was issued more than three years after the couple filed the 2015 return, so without the §6501(c) exception the assessment was time-barred.
• The underpayments were attributed to errors on the return, the taxpayers properly relied on advisers and return preparers, and because fraud was not established they were not liable for the §6663 penalty.

How the civil fraud penalty works

Under §6663(a), if any part of an underpayment required to be shown on a return is due to fraud, the IRS adds 75% of the portion of the underpayment attributable to fraud. Section 6663(b) then shifts the math: once the IRS establishes that any portion is attributable to fraud, the entire underpayment is treated as fraudulent, except for any portion the taxpayer proves, by a preponderance of the evidence, is not.

The IRS carries a heavy burden first. Per the Internal Revenue Manual on civil fraud (IRM 25.1.6), the penalty is asserted only when there is clear and convincing evidence that part of the underpayment was due to fraud, and that evidence must show the taxpayer’s intent to evade a tax they believed was owing. The IRM distinguishes intent from inadvertence, reliance on incorrect technical advice, a sincerely held difference of opinion, negligence or carelessness. A balance due alone is not enough; an affirmative act of fraud must also be present.

Joint returns get special treatment. Under §6663(c), the penalty does not apply to a spouse unless some part of the underpayment is due to that spouse’s own fraud. The IRM says intent must be established separately for each spouse, and one spouse’s fraud cannot be imputed to the other.

Why fraud controls the statute of limitations

The assessment periods in §6501 explain why the IRS fought for a fraud finding in Mitchell:

• Three years, the default. Under §6501(a), tax generally must be assessed within three years after the return was filed.
• Six years for large omissions. Under §6501(e), the period is six years for a substantial omission of more than 25% of income.
• No limit for fraud. Under §6501(c)(1), for a false or fraudulent return with intent to evade tax, the tax may be assessed at any time.

In Mitchell, the improper deductions were not omitted income, and the IRS could not prove intent to evade. That left only the three-year window, which had closed. Contrast that with the ordinary 20% accuracy-related penalty under §6662 for negligence or substantial understatement, which needs no proof of intent; see our guide to accuracy-related penalties after Clear Sky.

One caution for preparers: the fraud that opens the statute is not always the taxpayer’s. In Murrin v. Commissioner (No. 24-2037), a precedential opinion, the Third Circuit held that a return preparer’s intent to evade tax can trigger the unlimited assessment period under §6501(c)(1) even when the taxpayer did not share that intent, following the Tax Court’s Allen line and departing from the Federal Circuit’s BASR decision. The penalty under §6663 still turns on the taxpayer’s own fraud, but the open statute can reach the taxpayer anyway. Our post on ghost preparers and preparer responsibility covers why that matters for client intake.

What US CPA firms should tell clients

• Bad deductions are not fraud. Unsubstantiated §162 expenses can lose on the merits and draw a 20% penalty, but fraud needs clear and convincing proof of intent to evade. The badges the IRS looks for include concealment, false records, implausible explanations and a pattern of understatement.
• Records and candor are the defense. In Mitchell, documented reliance on advisers and return preparers mattered. Keep engagement letters, advice memos and the facts given to the preparer, and answer IRS questions consistently.
• Calendar the statute before you respond. Before agreeing to extend the assessment period or answering a late-stage exam, confirm the filing date, whether §6501(e) could apply, and whether the IRS is asserting fraud to reach an old year.
• Treat “strategy” packages with care. Advisor-designed structures with management fees and related entities are where substantiation failures cluster.

A short firm checklist

• For any exam that reaches back more than three years, pull the account transcript and filing date first, and ask which §6501 exception the IRS relies on.
• Map each proposed adjustment to its penalty basis: §6662 negligence or understatement, or §6663 fraud. The proof required and the defenses differ.
• Gather contemporaneous substantiation and adviser correspondence for every disputed deduction.
• Flag returns from preparers with known problems; under Murrin, their intent may keep the statute open.
• If a client may owe after all, plan early; see our guides to IRS audit rates and CP2000 notices and the IRS substitute for return after Jackson.

Where this fits for Tax Brew readers

Exam files need transcript pulls, timelines and substantiation binders, while your signing professionals own the judgment calls. If your firm needs extra capacity on that preparation work, outsourced tax prep and bookkeeping support for US CPA firms can help. Email info@taxbrewglobal.com.

Check current guidance: this article summarizes Mitchell v. Commissioner, T.C. Memo. 2026-102 (filed September 30, 2026; Judge Kerrigan; docket 9804-23, via Leagle), VitalLaw Federal Tax Day (October 1, 2026), Bloomberg Tax coverage (October 5, 2026), Murrin v. Commissioner (3d Cir., No. 24-2037), IRM 25.1.6 (Civil Fraud), and IRC §§6501(a), (c)(1) and (e), 6662 and 6663. Confirm the full opinions and current Code before advising. This is general information, not tax or legal advice. Sources as of 14 October 2026.