The $750,000 401(k) Mistake That Quietly Costs Beneficiaries $120,000 in Excess Taxes

A 58-year-old posted on Reddit’s tax forum last fall that his father had left him a $750,000 401(k), and the plan administrator wanted to know within 30 days whether to cut a check, open an inherited IRA, or set up installments. He took the check, figuring the 10-year rule gave him plenty of time to deal with taxes later. That single decision set up a tax bill roughly $120,000 higher than the path he could have chosen instead.

This is the most common, and most expensive, mistake on inherited 401(k) balances in the $500,000 to $1 million range. The fix is mechanical, and the window to get it right closes on December 31 of the year after death.

What the 10-year rule actually requires

Since the SECURE Act took effect, almost every non-spouse adult child who inherits a 401(k) or traditional IRA from a parent who died in 2020 or later must empty the account by December 31 of the tenth year after death. IRS final regulations that took effect in 2025 added an important layer: if the parent had already started required minimum distributions (RMDs), the beneficiary must also take annual distributions during years one through nine, in addition to fully emptying the account by year ten.

The mistake is reading “10 years” as permission to cash out immediately or to wait until year ten and pull everything at once. Both approaches compress hundreds of thousands of dollars of ordinary income into a single tax year, where the bracket cascade does its damage. Neither strategy is permitted when the decedent had already begun RMDs.

How $750,000 becomes $120,000 in extra tax

Consider a representative case: a married couple filing jointly with $200,000 in W-2 income, living in a state with a 6% top income tax rate. The inherited 401(k) is $750,000, and the decedent died in 2025.

Cash it out in year one and the household’s taxable income jumps to roughly $950,000. That amount pushes through the upper federal brackets, with the final slice spilling into the 37% top rate, which in 2026 begins at $768,700 for joint filers. Federal tax attributable to the inheritance comes in near $240,000. (The TCJA rate structure, including that 37% top rate, was made permanent by the One Big Beautiful Bill Act signed in July 2025, so these thresholds will not revert.)

Spread the same $750,000 evenly across ten years and annual taxable income rises to $275,000, keeping the top dollar still in the 24% bracket. Federal tax across the decade attributable to those distributions totals roughly $178,000.

The federal gap alone is about $62,500. State income tax on a lump sum adds another $25,000 to $35,000 in higher-tax states. Two years of Medicare IRMAA surcharges, triggered by the two-year MAGI lookback when the beneficiary or spouse turns 65, can add $4,000 to $10,000 per person. The 3.8% Net Investment Income Tax on taxable-account gains in that year adds further cost. All told, the avoidable tax bill lands near $120,000.

Why the lump-sum path is hard to undo

Three details make this mistake sticky. First, a 401(k) cashed out to the beneficiary cannot be rolled back into an inherited IRA. Second, the mandatory 20% federal withholding on plan distributions is only a deposit against tax owed; the beneficiary still reconciles the full bill at filing. Third, IRMAA shows up two years after the income year, often long after the proceeds have been spent or reinvested.

The behavioral pattern is well documented. According to Vanguard’s How America Saves 2025 report, about one-third of participants who leave a job take a cash lump sum rather than rolling the balance into another account, with cash-out rates running even higher for inherited balances handled outside an advisor relationship. The check is easy to ask for and almost impossible to take back.

Three moves before the December 31 deadline

1. Open an inherited IRA before any distribution is processed. A direct trustee-to-trustee transfer from the 401(k) preserves every distribution option, avoids the automatic 20% withholding, and starts the 10-year clock cleanly. Title the account exactly as the custodian requires, with the decedent’s name and your status as beneficiary.
2. Build a 10-year distribution schedule against your own bracket. If your household sits in the 22% or 24% bracket, fill those brackets each year and stop. Roughly equal annual withdrawals beat front-loading or back-loading in almost every case where future income is stable.
3. Confirm whether the decedent had started RMDs. If your parent was past age 73 and had taken even one RMD, you must take an annual RMD in years one through nine. Skipping a year triggers a 25% penalty on the missed amount, reduced to 10% if corrected within two years.

For balances above $500,000, a one-hour fee-only consult with a CPA or CFP to model the full 10-year drawdown against your projected income and IRMAA tiers pays for itself many times over.

Editor’s note: This article was updated to correct the Vanguard lump-sum cash-out figure from 29% to approximately one-third, in line with the How America Saves 2025 report, and to add context that the One Big Beautiful Bill Act made the 2026 federal tax rate structure permanent, along with the confirmed 37% bracket threshold of $768,700 for married couples filing jointly per IRS Rev. Proc. 2025-32.

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