10 Costly Medicare Mistakes High-Income Retirees Make

Medicare is more forgiving of mistakes if your income is modest. For retirees with $100K–$500K in annual income, the same errors can cost $1,000–$7,000 per year — sometimes permanently. These are the ten mistakes we see most often and what they actually cost.

Mistake 1: Claiming Social Security after 65 Without Stopping HSA Contributions First

When you claim Social Security retirement benefits at any age, Social Security automatically enrolls you in Medicare Part A and backdates enrollment up to 6 months. If you're 67 and claim SS today, Part A may be effective as of 6 months ago.

The trap: any HSA contributions you (or your employer) made during those backdated Part A months are excess contributions — subject to a 6% excise tax each year they remain in the account, plus income tax on any Medicare-covered expenses paid from those funds during the same period.¹

What it costs: Excess HSA contributions of $3,000–$4,300 (single; 2026 limit $4,400) over 6 months = 6% excise tax plus potential income tax on disqualified distributions. This is often discovered at tax time, not at enrollment.

The fix: If you plan to delay SS but eventually claim after 65, stop all HSA contributions at least 6 months before your intended Social Security start date. If your employer contributes to your HSA, update your plan enrollment to redirect those funds. See our detailed guide on the Medicare and HSA enrollment trap.

Mistake 2: Treating Medicare Advantage as Reversible

Enrolling in a Medicare Advantage plan is straightforward: during your Initial Enrollment Period or the Annual Enrollment Period (Oct 15–Dec 7), you choose an MA plan and you're in. What most enrollees don't realize is that switching back to Original Medicare with a Medigap supplement — the path many will want after a major diagnosis — requires passing medical underwriting in most states.

In 37+ states plus DC, Medigap insurers can ask about pre-existing conditions and deny coverage or charge much higher premiums outside of guaranteed-issue windows. Develop heart disease, diabetes, or cancer on an MA plan and you may find Medigap plans unavailable or unaffordable when you want to switch.²

What it costs: If you're denied Medigap and stay on MA with a serious condition: the MA $9,250 in-network annual out-of-pocket maximum (2026) vs. Medigap Plan G's near-zero cost-sharing. On an expensive chronic condition, that's potentially thousands per year for life.

The fix: Treat the Original Medicare + Medigap vs. MA decision as largely one-way in most states. A 12-month trial right exists when you're new to MA, but that window closes. If premium savings today could mean uncapped cost exposure later, model both scenarios. See Medigap guaranteed issue rights for the limited windows where switching back is protected.

Mistake 3: Not Knowing That 2026 Premiums Are Based on 2024 Income

IRMAA surcharges are based on your Modified Adjusted Gross Income from two years prior. Your 2026 Part B and Part D premiums are set by your 2024 tax return, which SSA receives from the IRS in the spring of 2025. A large income event in 2024 — a Roth conversion, business sale, equity payout, large IRA withdrawal — affects Medicare premiums in 2026, not 2024.³

The consequences are delayed and non-negotiable (barring a qualifying life-changing event). By the time the IRMAA notice arrives, the income that triggered it is already 18 months old and can't be undone.

What it costs: Tier 1: +$1,148/year. Tier 2: +$2,885/year. Tier 3: +$4,620/year. Per person. A couple where both spouses cross into Tier 2 pays an extra $5,770/year in combined IRMAA.

The fix: For any income event large enough to cross a tier threshold, model the 2-year lag explicitly. If you're age 63 planning a Roth conversion or equity exercise, you're setting your age-65 Medicare premium. See the complete 2026 IRMAA bracket table and the IRMAA MAGI explainer for what counts.

Mistake 4: Not Filing SSA-44 When You Retire Mid-Year

When you retire in, say, May 2024, your 2024 gross income is high — you worked five months and may have received significant investment or compensation income. Two years later, SSA uses that 2024 return to set your 2026 Medicare premiums. You're now fully retired on a lower income, paying surcharges based on income you no longer earn.

Most retirees don't know that retirement qualifies as a life-changing event for the SSA-44 form. Filing SSA-44 allows SSA to use your estimated current-year income instead of the 2-year-old return. You can reduce or eliminate IRMAA in the year it would otherwise apply — not two years later.⁴

What it costs to miss it: If retirement caused your income to drop from $180,000 (Tier 3, +$4,620/year) to $95,000 (no IRMAA), not filing SSA-44 costs $4,620 × 2 years = up to $9,240 in avoidable surcharges while the look-back catches up.

The fix: File SSA-44 with your local Social Security office as soon as you retire. Bring documentation of the income change. You don't have to wait for an IRMAA notice to arrive — you can be proactive. Full process at IRMAA appeal guide.

Mistake 5: Assuming COBRA Extends Your Part B Enrollment Window

When active employer coverage ends and you're eligible for COBRA continuation, many people assume the COBRA period (typically 18 months) gives them extra time to enroll in Part B. It does not.

The 8-month Special Enrollment Period for Part B is triggered by the end of active employer coverage — not COBRA. If your group health coverage ends April 30 and you elect COBRA that same day, your SEP clock started on April 30. Take 18 months of COBRA and enroll in Part B when COBRA ends: you're now 10 months past your SEP, and you have a permanent 10%/year Late Enrollment Penalty for every 12 months you were without Part B coverage outside your window.⁵

What it costs: A 10-month gap = 10% permanent LEP on top of the $202.90 standard premium = +$20.29/month for life. Over 20 years: approximately $4,870 in added premiums from one timing error.

The fix: Enroll in Part B within 8 months of when the employer plan that covered you (or your working spouse) actually ends — regardless of whether you take COBRA. COBRA is a bridge for Part A cost-sharing; it does not pause the Part B SEP clock. Full details at Medicare while working.

Mistake 6: Believing Muni Bond Interest Is Invisible to Medicare

Municipal bond interest is exempt from federal income tax. It doesn't appear in gross income and doesn't affect your ordinary income bracket. Many affluent retirees hold significant muni allocations specifically to reduce their tax bill. The assumption — often incorrect — is that it has no Medicare consequence.

IRMAA MAGI is defined as: AGI plus tax-exempt interest. Under IRC §1395r(i)(4), muni bond interest is explicitly added back to AGI in the IRMAA formula. A retiree holding $400,000 in munis at 3.5% generates $14,000 in tax-exempt income that counts 100% toward the $109,000 IRMAA threshold.⁶

What it costs: A retiree at $96,000 in other income who earns $15,000 in muni interest has IRMAA MAGI of $111,000 — $2,000 over the Tier 1 threshold. IRMAA: +$1,148/year. Federal income tax on the muni interest: $0. The tax savings from munis exists; the Medicare savings does not.

The fix: Model IRMAA MAGI including muni interest when planning your fixed-income allocation. If you're near a tier boundary, holding munis in a tax-deferred account (where interest isn't distributed annually) may produce a better combined outcome. See Bond interest and IRMAA for a full breakdown including CDs, Treasuries, I-bonds, and TIPS.

Mistake 7: Roth Conversion Without Checking the IRMAA Cliff

Roth conversions are a legitimate and often powerful planning strategy. But IRMAA is a cliff system, not a gradient: one dollar over a tier threshold costs the full tier increment. Converting past a boundary is not a small error — it's a large one that repeats annually until the look-back clears.

Example: A single retiree at $135,000 MAGI converts $5,000 more to Roth than planned, reaching $140,001. She's now in Tier 2 (+$2,885/year) instead of Tier 1 (+$1,148/year) two years later. The extra $1,737/year from one conversion overshoot costs $3,474 total across the two-year window — likely exceeding any tax advantage from the additional conversion in the first place.³

What it costs: Crossing any tier boundary mid-conversion costs $1,148–$1,737/year extra per person for 2 years before the look-back resets. Tier 1→2 overshoot: $3,474 over 2 years. Tier 2→3 overshoot: $3,470. These numbers are per person; couples can double them.

The fix: Model IRMAA MAGI explicitly when sizing Roth conversions. Build the tier thresholds ($109K, $137K, $171K, $205K single; $218K, $274K, $342K, $410K MFJ) into your Roth conversion plan as hard stops, not casual guardrails. See Roth conversion and IRMAA guide.

Mistake 8: Realizing 0%-Rate Capital Gains Without Knowing They Still Trigger IRMAA

Long-term capital gains are taxed at 0% for single filers with taxable income below approximately $49,450 in 2026. Many retirees "harvest" gains in low-income years specifically to take advantage of this zero-rate window. The problem: IRMAA is based on MAGI, not taxable income. Capital gains count 100% for MAGI regardless of the federal rate applied to them.

A retiree with $90,000 in other income (SS + pension) who realizes $22,000 in long-term gains pays $0 in federal capital gains tax on those gains — but her IRMAA MAGI rises from $90,000 to $112,000, crossing the $109,000 Tier 1 threshold and triggering $1,148/year in IRMAA two years later.⁷

What it costs: The $1,148/year IRMAA hit persists for two look-back years = $2,296 in extra premiums from a year of "tax-free" gains that weren't IRMAA-free. If you planned to harvest gains repeatedly over multiple years at the 0% rate, each year near the threshold carries this risk.

The fix: When planning gain harvesting, use MAGI as your ceiling — not taxable income. The gap between your other income and the $109,000 IRMAA threshold is the real amount of gains you can realize safely. See Capital gains and IRMAA guide with worked examples and an interactive calculator.

Mistake 9: Filing Married-Filing-Separately for Other Tax Reasons

Couples occasionally file taxes separately to isolate one spouse from the other's audit exposure, to claim a deduction that phases out at higher joint AGI, or for other household reasons. The IRMAA consequence is severe: married-filing-separately brackets are nearly all punitive. Tiers 1 through 3 don't exist for MFS filers. MAGI above $109,000 on a separate return jumps directly to Tier 4, skipping the gradations that would have applied on a joint return.³

What it costs: A spouse with $130,000 in retirement income filing separately faces a Tier 4 IRMAA surcharge (+$6,355/year). Filing jointly at $260,000 combined MAGI would be Tier 2 (+$2,885/year each). The filing-status choice alone creates a $3,470/year difference per person, for potentially thousands of dollars of annual penalty just for filing separately.

The fix: Before choosing to file separately, model the IRMAA consequence explicitly — not just the income tax consequence. In most cases, the IRMAA cost exceeds the income tax benefit of filing separately. If there's a legitimate reason to file separately (e.g., income-driven student loan repayment for a still-working spouse), the calculation still needs to include Medicare surcharges.

Mistake 10: Taking Deferred Compensation as a Lump Sum Without IRMAA Modeling

Executives with non-qualified deferred compensation (NQDC) plans under IRC §409A often face a default payout structure that distributes the full account in the year following separation from service. A $400,000 NQDC payout year pushes MAGI into Tier 4 or Tier 5, triggering up to $6,936/year in IRMAA per person — starting two years later and continuing for as long as the look-back is elevated.

Installment elections — spreading the payout over 5, 10, or 15 years — must be made years in advance under §409A rules. After age 65, a 12-month advance re-election with a mandatory 5-year delay is sometimes still possible, but the window is narrow and easy to miss. The tax and Medicare savings from spreading a large NQDC payout can be substantial.⁸

What it costs: A $500,000 lump-sum payout year: single filer MAGI ~$600,000 = Tier 5 = +$6,936/year × 2 look-back years = $13,872 in extra IRMAA from the distribution structure alone. Compare installments of $100,000/year over 5 years: depending on other income, may stay at Tier 1-2 for each year rather than Tier 5 for one year.

The fix: Model your §409A payout structure alongside your anticipated retirement income before you separate from service. The installment vs. lump-sum decision has both income tax and Medicare consequences. See Deferred compensation and IRMAA guide for worked scenarios and the §409A re-election rules.

Common thread: income-aware Medicare planning

All ten of these mistakes share a common cause: Medicare and financial planning being done in separate silos. An insurance agent handles Medicare enrollment; a CPA handles income taxes; a financial advisor handles investments. None of them necessarily owns the IRMAA look-back calculation across all income sources.

The retirees who avoid these mistakes typically have an advisor who explicitly models Medicare MAGI alongside retirement income projections — tracking what income in 2024 does to 2026 Medicare premiums, what a Roth conversion in 2025 does to 2027 premiums, and what a business sale or equity event does two years forward. This isn't Medicare planning or tax planning separately; it's the same calculation.