1. Missing your Initial Enrollment Period for Part B
Your Initial Enrollment Period (IEP) is the seven-month window around your 65th birthday — the three months before, the month of, and the three months after. If you don't have creditable employer coverage and you don't enroll in Part B during this window, you pay a 10% lifetime penalty for every full 12-month period you delayed. Forever. The penalty is added to your Part B premium every month for the rest of your life.
Real math: in 2026, the standard Part B premium is $202.90/month. A two-year delay = 20% penalty = $40.58/month forever. Over 20 years of retirement, that one missed window costs $9,739.
How to avoid it: if you're approaching 65 and you don't have employer health coverage from a company with 20+ employees, sign up for Part B during your IEP. Period. If you already missed it, the General Enrollment Period runs January 1 - March 31 each year, and the new "earlier coverage" rule (effective 2023) means coverage starts the month after you enroll instead of July 1.
2. Missing your one-time Medigap guaranteed-issue window
When you first enroll in Medicare Part B, you have a one-time, six-month Medigap Open Enrollment Period during which insurance carriers cannot reject you, charge you more, or apply preexisting condition exclusions. Outside that window, in most states, carriers can subject you to medical underwriting — meaning if you've had cancer, heart disease, or diabetes, they can charge you double, triple, or refuse coverage entirely.
This window matters because Plan G coverage is federally standardized — every carrier covers identical benefits, so you're shopping on price alone. But miss the window and lose the ability to shop on price; you may be locked into whatever carrier accepts you, potentially paying $200/month more for the same coverage.
How to avoid it: if you're going on Original Medicare (not Medicare Advantage), enroll in a Medigap plan within six months of your Part B effective date. Use this window to pick the cheapest Plan G or N carrier for your state — you can run our comparison tool to see the spread.
3. The Part D late enrollment penalty
If you don't have creditable prescription drug coverage from another source (employer plan, VA, etc.) and you go more than 63 days without Part D after your IEP, you pay a permanent penalty of 1% of the national base premium per month uncovered. The penalty is recalculated each year as the national base premium changes — and it's added to your Part D premium for the rest of your life.
Two years uncovered = 24% penalty. The 2026 national base premium is $36.78, so 24% = $8.83/month forever. Over 20 years, that's $2,119 in penalties for two years of "I don't take any meds, why would I pay for drug coverage?"
How to avoid it: enroll in some Part D plan during your IEP, even if you don't take medications. The cheapest Part D plans are around $0-15/month. Compare the lifetime penalty math against the premium — Part D is one of the rare cases where buying "useless" coverage is cheaper than going without.
4. Assuming COBRA or retiree coverage counts as creditable
COBRA does not count as "current employer coverage" for Part B enrollment purposes. Neither does most retiree health insurance. People assume their COBRA coverage protects them from the Part B late penalty — it doesn't. The penalty clock starts when you turn 65, regardless of what other coverage you have.
This trips up an enormous number of people who lose their job at 64 or 65, get COBRA for 18 months, then enroll in Medicare and discover they owe a permanent late-enrollment penalty for the entire COBRA period.
How to avoid it: if you turn 65 and you're on COBRA or retiree coverage, enroll in Medicare Part B during your IEP. You can keep COBRA as secondary coverage, but Medicare needs to be primary. Confirm with your employer's benefits administrator in writing whether your coverage qualifies as "creditable" — get the letter.
5. Working past 65 with the wrong employer size
If your current employer has fewer than 20 employees, Medicare is your primary insurance starting at 65 even if you stay on the company plan. The company plan becomes secondary, and if you haven't enrolled in Medicare you have a massive coverage gap on every claim. We've seen retirees discover this after a $50,000 hospital bill.
If your employer has 20 or more employees, you can defer Medicare without penalty as long as your group coverage is creditable. Keep the annual letter from your insurer confirming creditable coverage — you'll need it when you eventually enroll.
How to avoid it: ask HR, in writing, whether your employer has 20+ employees and whether your plan is creditable for Medicare deferral. If you switch jobs after 65 to a smaller company, enroll in Medicare immediately. See our working past 65 guide for the full decision tree.
6. Picking Medicare Advantage based on the $0 premium
Most Medicare Advantage plans advertise a $0 monthly premium. What they don't advertise prominently: the in-network out-of-pocket maximum is typically $4,000-$8,000 per year, your hospital might not be in-network in 2027 even if it is in 2026, and over 70% of MA plans in some states require prior authorization for specialist visits and most procedures (compared to less than 10% in the lowest-friction states).
The Commonwealth Fund's 2024 Value of Medicare survey found that 22% of MA enrollees experienced a delay in care due to required approvals, vs. 13% of beneficiaries on Original Medicare. The "$0 premium" carries a real cost — it's just paid in friction and denied claims rather than monthly bills.
How to avoid it: compare the total expected cost — premium + drug copays + IRMAA + a realistic estimate of your medical out-of-pocket spending — between Medicare Advantage and Original Medicare + Plan G + Part D. Our comparison tool shows both paths side-by-side with the math.
7. Letting your plan auto-renew without reviewing the ANOC
Every year in late September, your Medicare Advantage or Part D plan sends an Annual Notice of Change (ANOC) — a 25-50 page document detailing what's changing for next year: premium, deductible, drug formulary, network, prior auth rules. Most beneficiaries glance at it and let the plan auto-renew.
That's how you wake up in February having paid $400 more in copays on a med that moved from Tier 2 to Tier 4. The plan didn't lie — they sent you the ANOC. They just buried the change on page 23.
How to avoid it: use the Annual Election Period (Oct 15 - Dec 7) to re-shop every year. Plans change. Premiums change. Drug coverage changes. The plan that was best for you in 2024 may not be best in 2026. (Or sign up for our Watchdog and we re-run the comparison for you.)
8. Enrolling in Part A while contributing to an HSA
Once you enroll in Medicare Part A — even premium-free Part A, even if you don't enroll in Part B — you can no longer contribute to a Health Savings Account. Worse: Part A enrollment is retroactive up to six months when you sign up after your IEP, so HSA contributions from those six months become excess contributions subject to a 6% IRS excise tax.
How to avoid it: if you're working past 65 and contributing to an HSA, stop HSA contributions at least six months before you plan to enroll in any Medicare program. If you're already collecting Social Security, you'll be auto-enrolled in Part A — there's no way to opt out without disclaiming Social Security entirely.
9. Not appealing IRMAA when your income drops
IRMAA — the Income-Related Monthly Adjustment Amount — uses your tax return from two years ago. Retired in 2024 with a $400,000 final-year compensation? In 2026 you're paying IRMAA based on $400k of MAGI even though you're now living on $80k of Social Security and pension.
The fix is form SSA-44, which lets you appeal IRMAA for "life-changing events" including: marriage, divorce, death of spouse, work stoppage, work reduction, loss of income-producing property, loss of pension, employer settlement payment. Most retirees qualify under "work stoppage" the year they retire.
How to avoid it: if your current income is meaningfully lower than your tax return from two years ago, file SSA-44 the month you receive your IRMAA notice. Don't wait until next year — the savings start the month SSA processes your form.
10. Buying the wrong Medigap plan letter
Medigap plans are federally standardized. Every Plan G covers the exact same things; every Plan N covers the exact same things; etc. But the plans themselves differ in coverage and price. Plan G covers the Part B deductible; Plan N doesn't (you pay it yourself — $257 in 2026). Plan N has small office-visit copays; Plan G doesn't. Plan F is closed to anyone who became Medicare-eligible after January 1, 2020.
For most new enrollees, the choice is between G and N. G has higher premiums but no copays; N has lower premiums but a $20 office-visit copay and $50 ER copay (waived if admitted). The break-even depends on how often you see a doctor.
How to avoid it: understand the trade-off before you enroll. If you visit specialists routinely (rheumatologist, cardiologist, oncologist), Plan G usually wins. If you're healthy and only see your PCP a few times a year, Plan N often beats G by $200-$400/year. Our comparison tool models both.
The mistakes are systemic — the system was built this way
Most Medicare mistakes happen because the rules were designed by different agencies at different times for different reasons. Part B was added in 1965. Part C in 1997. Part D in 2003. The Medigap standardization happened in 1990. The IRMAA appeal form was added in 2007. Each layer has its own deadlines, penalties, and appeal paths.
The agents and brokers who help you navigate this are paid commissions by specific carriers — they have a structural conflict we don't share. We built the comparison tool because nobody else could honestly rank every carrier and every plan against your specific situation. If that's helpful, run it. If you'd rather just avoid the mistakes above, the guide is yours — bookmark it.