A single Medicare enrollment mistake can cost a retiree thousands of dollars every year for the rest of their life — not just one season of paperwork. The Part B late enrollment penalty adds 10% of the premium for every full 12-month period you delayed, and you pay that higher premium for as long as you hold Part B (CMS). As a retirement income advisor in New Hampshire, I watch clients lose real income to penalties they never saw coming — often because they delayed a decision they assumed could wait.
These mistakes are permanent. They are not correctable with a phone call or a late payment. That's why understanding the enrollment window before you turn 65 matters more than any other retirement planning step you'll take this year. Here are the seven most costly mistakes I see, and exactly how to avoid each one.
Mistake 1: Missing the Initial Enrollment Period (IEP)
The Initial Enrollment Period is the 7-month window that opens 3 months before the month you turn 65, includes your birthday month, and closes 3 months after — and it is the only penalty-free window most people get (CMS). Coverage begins the month after you enroll during the IEP, so signing up early in that window means your protection starts sooner, with no gap.
The mistake I see most often is treating that window as a loose deadline. If you miss the IEP entirely, you fall to the General Enrollment Period, which runs January 1 through March 31 each year — and your Part B coverage won't start until the month after you enroll (CMS). That can leave you uncovered for months and, worse, starts the clock on a penalty.
If you're still working with employer coverage based on current employment, delaying is fine — but only because a Special Enrollment Period (SEP) protects you. The key condition: the coverage must be based on your own or your spouse's current employment, not on any other arrangement. I walk every client through this distinction before they decide to wait.
Mistake 2: Ignoring the Part B Late Enrollment Penalty
The Part B penalty adds 10% of the premium for every full 12-month period you were eligible but didn't enroll, and you pay that higher premium for as long as you hold Part B (CMS). That is a lifetime surcharge, not a one-time fine. The penalty is measured against the current standard premium, so it grows right alongside inflation.
Consider what two years of delay costs. If the standard Part B premium is roughly $202.90 in 2026, a 20% penalty adds about $40 a month — more than $4,800 over a decade, with no mechanism to ever reduce it while you stay enrolled (SSA). Many retirees shrug off the delay as a paperwork mistake; financially, it is one of the most expensive decisions in retirement planning.
The only reliable protection is a qualifying SEP. Enrollment during a valid SEP carries no penalty. Without one, the surcharge compounds silently for years — which is exactly why I advise treating the IEP as a firm commitment, not an option.
Mistake 3: Overlooking Part D and 'Creditable Coverage'
Go 63 or more days without creditable prescription coverage and you owe a permanent Part D penalty of 1% of the national base beneficiary premium for every uncovered month (CMS). The surcharge stays with you for life once applied, so the drug coverage decision is a one-time, permanent choice — not a reversible paperwork slip.
"Creditable coverage" is the technical term that decides whether you're safe. A plan is creditable only if its prescription coverage is expected to pay at least as much as the standard Medicare prescription coverage — a standard CMS sets each year. Your employer or COBRA administrator must send you a Notice of Creditable Coverage annually stating whether your drug plan qualifies; keep every one of those notices as proof.
The common trap is assuming any drug coverage protects you. Retiree health plans and many private drug benefits are not creditable, which means a retiree who trusts their old plan and skips Part D can quietly rack up a lifelong penalty. Check the notice, and if there's any doubt, enroll in a Part D plan during your IEP — the penalty math is never worth the gamble.
Mistake 4: Assuming COBRA Counts as Coverage
COBRA does not count as coverage based on current employment, so delaying Medicare while on COBRA can trigger the lifelong 10% Part B penalty (CMS). COBRA lets you temporarily keep employer or union coverage after your job ends, but that status difference — current employment versus coverage kept after employment — is exactly what separates a safe delay from a costly one.
Here's the mechanism: the Special Enrollment Period that protects you from penalties exists specifically for people covered under a group health plan based on current employment — your own or your spouse's. COBRA is coverage you keep after employment ends, so it falls outside that definition. When your COBRA ends, you won't qualify for the SEP, and the penalty clock is already running.
The right move if you're turning 65 while on COBRA: enroll in Medicare during your IEP and drop COBRA, rather than waiting. COBRA premiums are also typically far higher than Medicare Part B — so waiting is both a penalty trap and a cash drain. Coordinate your effective dates with your benefits administrator so you never lose coverage.
Mistake 5: Missing the IRMAA Surcharge Surprise
IRMAA — the Income-Related Monthly Adjustment Amount — is an extra premium that higher-income retirees pay on top of the standard Part B and Part D premiums, and it catches many retirees off guard in their first year of Medicare. Social Security uses the Modified Adjusted Gross Income (MAGI) from your tax return two years prior to set your surcharge, so a high-earning final working year can spike your premium unexpectedly.
For premium year 2026, a single filer with MAGI more than $109,000 but not more than $137,000 pays 35% of the true Part B cost, rising through five levels to 85% at $500,000 or more (SSA). The surcharge applies to both Part B and prescription drug coverage. If you had an unusually good income year right before retirement, that single year's tax return can inflate your Medicare premiums for a full year.
The good news: this is fixable. If your income drops due to a life-changing event — retirement (work stoppage or reduction), divorce, death of a spouse, or loss of pension income — you can file an SSA-44 form to request a new determination based on your current, lower income (SSA). I tell clients to file this the moment they retire, not after a full year of overpaying.
Mistake 6: Letting IRMAA Appeals Time Out
You have only 60 days from the IRMAA determination notice to request reconsideration, and the date received is presumed to be 5 days after the notice date unless you can show otherwise (SSA). Missing that window locks in a year of surcharges you could have removed — the single most common IRMAA failure I see in practice.
Here's the distinction that matters: for a life-changing event, you don't need a formal appeal at all. The new initial determination process (via the SSA-44) is the faster, cleaner path, and there's no need to appeal the prior determination if you qualify for it (SSA). You can even file an appeal and request a new initial determination at the same time if you're within the appeal period.
Either way, the clock matters. Act on the notice the week you receive it. The appeals path runs through four levels — reconsideration, a hearing before an Administrative Law Judge, the Medicare Appeals Council, then federal court — so a timely start saves you from fighting an uphill year-long battle over a surcharge you could have removed with one form.
Mistake 7: Failing to Recheck Your Part D Plan Each Year
Once you've enrolled, the work isn't done. Prescription drug plans change their formularies, premiums, and pharmacy networks every single year, and the plan that suited you at 65 can quietly become a poor fit by 67. The Annual Coordinated Election Period runs every fall, letting you switch Part D plans or Medicare Advantage plans for the following year (CMS).
The trap is assuming your plan stays constant. A drug that was covered last year may be moved to a higher tier or dropped entirely, leaving you to pay full price at the pharmacy. I advise a simple annual ritual: compare your current prescriptions against your plan's new formulary each fall, and check whether a competitor plan covers the same drugs at a lower total cost.
Medicare's plan finder tool makes this comparison straightforward, and working with an independent advisor adds a layer of protection — because the costliest Medicare mistakes are rarely the ones you make once. They're the ones you repeat every year without noticing.
The Bottom Line
Medicare enrollment is a one-time decision with lifetime consequences. The penalties are permanent, compounding, and rarely reversible — which is exactly why the window before you turn 65 deserves as much planning attention as any retirement income decision.
If you take nothing else from this, remember the three rules that prevent nearly every costly mistake: enroll during your 7-month Initial Enrollment Period, keep proof of creditable coverage if you delay Part D, and file the SSA-44 the moment a life-changing event lowers your income. Get those three right, and the expensive surprises — the 10% Part B penalties, the lifelong Part D surcharges, and the IRMAA overpayments — never touch your retirement income.
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