Retirement Tax Mistakes Most People Don’t See Coming

Most people retire thinking their biggest financial battle is over. They’ve saved diligently, watched their accounts grow, and now it’s time to enjoy the fruits of decades of discipline. Then the first big withdrawal hits—and so does the tax bill they never saw coming. The uncomfortable truth is that a retirement account balance is not the same as spendable money. A significant portion of most retirees’ wealth—particularly in traditional 401(k)s and IRAs—has never been taxed. And the government has been patient. Now it’s their turn. What makes retirement taxes especially treacherous isn’t just their size. It’s their invisibility. The mistakes below aren’t obscure technicalities. They’re predictable traps that catch financially literate people every year, simply because no one explained how the pieces connect.

Table of Contents

1. 1. The hidden tax bill on 401(k) withdrawals

1. 1. The RMD ambush — forced withdrawals at the worst time

1. 1. Yes, Social Security is taxable — up to 85%

1. 1. The Medicare surcharge nobody mentions: IRMAA

1. 1. The Roth conversion window and why most people miss it

Mistake #1: The Hidden Tax Bill on 401(k) Withdrawals

Here’s a number that shocks nearly every pre-retiree: if you have $1,000,000 in a traditional 401(k) or IRA, you may only have access to somewhere between $600,000 and $780,000 after taxes—depending on your income, state, and how you withdraw. The reason is simple: every dollar in a traditional 401(k) was deposited pre-tax. The deal was to defer income now and pay taxes when you take the money out. The problem is that at retirement, many people are surprised by how high those taxes actually are.

It’s ordinary income, not a capital gain

The single most important thing to understand about 401(k) withdrawals: they are taxed as ordinary income, not as long-term capital gains. That means your withdrawals are stacked on top of all your other retirement income—Social Security, pensions, part-time work—and taxed at your marginal rate, not the preferential 15–20% long-term capital gains rate.

Many retirees plan their budgets around a 12% federal tax rate. But when a large 401(k) withdrawal gets stacked on top of Social Security income, the effective marginal rate on that last dollar withdrawn can jump to 22%, 24%, or higher—plus state income tax in most states.

How a $60,000 withdrawal actually looks

• • Gross withdrawal: $60,000

• • Federal taxes (22% bracket): −$8,400

• • State income tax (avg 5%): −$3,000

• • Net spendable: ~$48,600 — about 19% gone to taxes

State taxes: an often-overlooked variable

State tax treatment of retirement income varies dramatically. Some states—like Illinois, Mississippi, and Pennsylvania—fully exempt 401(k) and IRA withdrawals. Others, like California, tax them at full ordinary income rates. If you’re considering relocating in retirement, this difference alone can be worth tens of thousands of dollars over a 20-year retirement.

What to do about it

• • Model your actual effective tax rate at different withdrawal levels—not just your marginal rate

• • Consider spreading large withdrawals across multiple tax years to stay in lower brackets

• • Compare tax treatment before choosing your retirement state

• • Work with a financial planner to build a tax-efficient withdrawal sequence (Roth first, then taxable brokerage, then traditional accounts)

Mistake #2: The RMD Ambush — Forced Withdrawals at the Worst Time

Even if you don’t need money from your retirement accounts, the IRS eventually forces you to take it out anyway. These are called Required Minimum Distributions (RMDs)—and they represent one of the most disruptive tax events in retirement planning.

How RMDs work

Starting at age 73 (raised from 72 by the SECURE 2.0 Act), you must withdraw a minimum amount from your traditional IRA, 401(k), 403(b), and most other tax-deferred accounts each year. The amount is calculated by dividing your account balance by an IRS life expectancy factor. There is no opting out. Fail to take your RMD and you face a penalty of 25% of the amount you should have withdrawn.

RMD amounts by age — example with $800,000–$920,000 balance

• • Age 73, $800,000 balance: ~$30,189 required withdrawal (~$6,642 in taxes at 22%)

• • Age 75, $875,000 balance: ~$35,569 required withdrawal (~$7,825 in taxes)

• • Age 80, $920,000 balance: ~$45,545 required withdrawal (~$10,020 in taxes)

• • Age 85, $880,000 balance: ~$55,000 required withdrawal (~$12,100 in taxes)

The compounding problem nobody talks about

The real RMD ambush isn’t a single year—it’s the trajectory. As your balance grows through your late 60s and early 70s while you delay Social Security and don’t yet need withdrawals, you’re accumulating a larger deferred tax liability. Then RMDs kick in, often pushing retirees into higher brackets and triggering the Social Security taxation threshold and the IRMAA Medicare surcharge simultaneously.

Many retirees in their late 60s are unknowingly building a tax bomb—one that detonates precisely when they stop working and lose their ability to easily manage the income it creates.

The spousal inheritance trap

When one spouse dies and leaves a large IRA to the surviving spouse, the survivor consolidates both accounts—often dramatically increasing their RMDs. Now a single filer with higher marginal rates, they may face a tax shock they never anticipated.

What to do about it

• • Project your RMDs at ages 73, 80, and 85 using your current balance and expected growth rate

• • Consider strategic Roth conversions in your 60s to reduce the future taxable balance

• • Qualified Charitable Distributions (QCDs) allow you to give up to $105,000 annually directly from an IRA to charity—counting toward your RMD without being included in taxable income

• • Work with an advisor who models the interaction between your RMDs, Social Security, and Medicare premiums together

Mistake #3: Yes, Social Security Is Taxable — Up to 85%

This is perhaps the most common retirement tax misconception: many people believe Social Security benefits are tax-free. They aren’t. Depending on your “combined income,” up to 85% of your Social Security benefits can be included in your taxable income.

How combined income works

The IRS uses a measure called “combined income” (or provisional income) to determine how much of your Social Security is taxable. It’s calculated as: Adjusted Gross Income + nontaxable interest + 50% of your Social Security benefits

Social Security taxation thresholds

For single filers:

• • Combined income below $25,000 → 0% of benefits taxable

• • Combined income $25,000–$34,000 → up to 50% of benefits taxable

• • Combined income above $34,000 → up to 85% of benefits taxable

For married filing jointly:

• • Combined income below $32,000 → 0% of benefits taxable

• • Combined income $32,000–$44,000 → up to 50% of benefits taxable

• • Combined income above $44,000 → up to 85% of benefits taxable

These thresholds were set in 1983 and 1993 and are not indexed for inflation. A combined income of $34,000 was a comfortable middle-class income in 1993. Today it catches tens of millions of retirees. Congress has shown no urgency to adjust these figures.

The phantom tax bracket: the 40.7% zone

For certain income ranges, the effective marginal rate on additional income can be dramatically higher than your stated bracket. This happens because each additional dollar of income causes more Social Security benefits to become taxable—essentially taxing a dollar twice simultaneously. In the range where Social Security taxation is phasing in at the 22% bracket, retirees can face an effective marginal rate of approximately 40.7%—without appearing to be in that bracket at all.

What to do about it

• • Model your combined income carefully before deciding when to take Social Security and how much to withdraw from traditional accounts in the same year

• • Consider Roth conversions before age 70 to reduce future traditional account withdrawals and therefore future combined income

• • Roth IRA distributions do not increase combined income — traditional IRA withdrawals do

• • Delaying Social Security increases your benefit but may also increase the amount subject to taxation — model both sides

Mistake #4: The Medicare Surcharge Nobody Mentions — IRMAA

Most people expect Medicare Part B to cost a fixed monthly premium. What they don’t know is that if your income exceeds certain thresholds, you pay significantly more — a surcharge officially called the Income-Related Monthly Adjustment Amount, or IRMAA. IRMAA applies to both Part B (medical coverage) and Part D (prescription drug coverage). And the kicker: it’s based on your income from two years ago — meaning a one-time high-income event like a large Roth conversion, a property sale, or an unusual withdrawal can elevate your Medicare premiums for an entire year.

2025 IRMAA brackets (Part B monthly premium)

• • Individual income ≤ $106,000 / Joint ≤ $212,000: $185/month — no surcharge

• • Individual $106,001–$133,000 / Joint $212,001–$266,000: $259/month (+$888/yr per person)

• • Individual $133,001–$167,000 / Joint $266,001–$334,000: $370/month (+$2,220/yr)

• • Individual $167,001–$200,000 / Joint $334,001–$400,000: $480.90/month (+$3,550/yr)

• • Individual $200,001–$500,000 / Joint $400,001–$750,000: $591.90/month (+$4,882/yr)

• • Individual above $500,000 / Joint above $750,000: $628.90/month (+$5,330/yr)

Part D surcharges are additional. Brackets adjust annually for inflation.

The $1 cliff problem

IRMAA brackets work as “cliff” thresholds, not gradual phase-ins. One dollar of income over a threshold moves you into the next bracket — adding hundreds or thousands of dollars in annual premiums for both you and your spouse. A $1,000 Roth conversion decision could cost or save $1,776 in combined Medicare premiums for a married couple.

IRMAA is determined by your income from two years prior. A Roth conversion, large IRA withdrawal, or real estate transaction in 2024 will affect your 2026 Medicare premiums — with no warning and no ability to undo the triggering transaction.

You can appeal IRMAA

If your income dropped significantly due to a “life-changing event” — retirement, reduced work hours, divorce, death of a spouse, or loss of income-producing property — you can request that Social Security recalculate your IRMAA using more recent income. File SSA Form SSA-44. This can eliminate or reduce surcharges in the year you need it most.

What to do about it

• • Build IRMAA thresholds into your annual income planning — treat them like the tax brackets they functionally are

• • Spread large Roth conversions over multiple years to avoid crossing a single threshold in one shot

• • Model the two-year lookback: plan for what your Medicare premiums will be in 2026 based on 2024 income

• • If you retire mid-year or your income drops significantly, file SSA-44 to appeal your IRMAA determination

Mistake #5: The Roth Conversion Window — and Why Most People Miss It

Every mistake listed above has a common thread: they’re driven by having too much money in tax-deferred accounts. The Roth conversion window is the strategy that attacks that problem directly — but only works during a specific window of time that most retirees fail to act on.

What the window is

For many retirees, there’s a period between when they stop working (income drops) and when RMDs begin at age 73, Social Security starts, and Medicare kicks in — often ages 60 to 72. During this period, taxable income can be unusually low, placing retirees in the 10%, 12%, or even 0% tax brackets. Roth conversions during this window allow retirees to pay tax at today’s lower rates on money transferred from a traditional IRA to a Roth IRA. Once in the Roth, that money grows and distributes tax-free forever — with no RMDs required during the account owner’s lifetime.

The window between retiring and starting Social Security and Medicare is often the lowest-income — and therefore lowest-tax — period in a person’s financial life. Most people waste it by doing nothing.

The math of a strategic conversion

• • Converting $50,000/year at 12% bracket: Tax cost ~$6,000/year

• • Over 10 years (ages 62–72): ~$500,000 moved to Roth

• • RMD reduction at age 73: ~$18,900 less in forced annual withdrawals

• • Projected lifetime tax savings: $80,000–$200,000+ depending on rates, growth, and lifespan

Roth conversions interact with every other mistake on this list

A successful conversion strategy doesn’t just reduce future tax bills in isolation. It lowers your future RMDs — reducing the chance of being pushed into higher brackets. It reduces your combined income — potentially keeping more Social Security tax-free. And it helps manage IRMAA thresholds in future years. The strategies compound on each other.

Common conversion mistakes to avoid

• • Converting too much in one year: Pushing income across IRMAA thresholds or into higher brackets can erase the conversion’s benefit

• • Paying the tax from the converted funds: Pay conversion taxes from a taxable account, not from the IRA itself — this maximizes what stays in the Roth

• • Waiting too long to start: Conversions after Social Security and Medicare begin are significantly harder to execute efficiently

• • Ignoring state taxes: Some states don’t recognize Roth conversions favorably — check your state before converting large amounts

How to execute a Roth conversion strategy

1. 1. Project your income trajectory from retirement through age 80 — including Social Security, RMDs, pensions, and expected Roth distributions

1. 1. Identify the low-income window between retirement and your RMD start date. Map exact tax brackets and IRMAA thresholds for each year

1. 1. Calculate the optimal annual conversion amount — typically the amount that fills your current bracket without crossing into the next

1. 1. Model the 10-year impact on future RMDs, Social Security taxation, and Medicare premiums before committing

1. 1. Fund the tax bill from outside the IRA using a taxable brokerage or savings account — don’t let the IRS take its share from the converted funds

The Bottom Line: Five Traps, One Strategic Window

These five mistakes don’t operate independently. They compound. An unplanned RMD pushes up your combined income, which makes more Social Security taxable, which triggers an IRMAA surcharge. The Roth conversion window is the strategic lever that can defuse all of them — but only if you act before you need to. Quick recap:

• • 401(k) withdrawals: Every dollar is ordinary income. Plan your bracket, not just your balance.

• • RMD ambush: Forced income at 73+ can detonate tax liability you spent a decade accumulating.

• • Social Security tax: Up to 85% of benefits are taxable. Thresholds haven’t moved since 1993.

• • IRMAA surcharge: $1 over a threshold = hundreds in Medicare premiums. The two-year lookback bites hardest.

• • Roth window: Ages 60–72 are your lowest-tax years. Converting now reduces every other risk.

Questions? Reach out, and we’ll get in touch to find a time to learn more about the particular tax risks you may be facing.