Medicare Enrollment — The 5 Mistakes That Cost Retirees Thousands

Most Medicare enrollment mistakes are permanent. A late Part B sign-up adds 10% to your premium forever. The Medigap window closes at 65 and never reopens. The COBRA trap locks people out of coverage they needed. Here's what the mistakes actually cost — and how each one happens to otherwise-organized people.

I've now watched four close friends navigate Medicare enrollment. One paid the late penalty for a decade. One got locked out of Medigap when she was 67 and couldn't pass underwriting. One was in the COBRA trap and had a two-month gap in coverage that cost $14,000 out of pocket for a hospital stay. The fourth did it right, partly by luck and partly because she'd seen what happened to the rest of us.

The common thread: Medicare's rules are counterintuitive, poorly communicated, and permanent in their penalties. The Social Security office will send you a form. They will not explain what happens if you file it three months late.

The Medicare Enrollment System (Brief Overview)

Before the mistakes, a quick orientation.

Medicare has parts: - Part A: Hospital coverage. Most people qualify for free. You should enroll at 65 regardless. - Part B: Outpatient care (doctors, tests, procedures). Premiums apply. This is where late penalties hit. - Part C (Medicare Advantage): Private insurance that bundles A+B, often adds dental/vision. Alternative to original Medicare. - Part D: Prescription drug coverage. Late penalties apply here too. - Medigap (Supplement): Private policies that pay what Medicare doesn't cover. Separate from Parts A-D.

Your Initial Enrollment Period (IEP) is the 7-month window around your 65th birthday: 3 months before, your birth month, 3 months after.

If you miss this window without a qualifying exception, penalties begin.

Mistake #1: Missing the Initial Enrollment Period for Part B

This is the most expensive and most common mistake.

The rule: If you don't enroll in Part B during your IEP and don't have qualifying employer coverage, you pay a late enrollment penalty of 10% per year you delayed. This penalty is permanent and added to your monthly premium for life.

What it actually costs:

The 2026 Part B standard premium is $185.00/month.

If you delay 2 years: $185 + 20% = $222/month. Every month. For the rest of your life. If you delay 5 years: $185 + 50% = $277.50/month. Permanently.

A person who delays 3 years and lives 20 more years pays an extra $13,320 in penalties ($185 × 30% × 12 months × 20 years). That's for one mistake made at age 68 that follows them to 88.

Why it happens: People assume they can "wait until they need it." Medicare doesn't work that way. It's an insurance system — you enroll when you're eligible, not when you're sick.

The exception: If you're enrolled in a qualifying employer group health plan (through your own or your spouse's employer) when you turn 65, you have a Special Enrollment Period (SEP) — you can delay Part B while you have that coverage and enroll within 8 months of losing it, with no penalty. This is the only clean exception.

COBRA does not count. See Mistake #3.

Mistake #2: Missing Part D Enrollment and Paying the Lifetime Penalty

Same logic as Part B, smaller numbers, same permanence.

The rule: If you don't enroll in Part D (prescription drug coverage) when you first become eligible and don't have qualifying drug coverage (employer plan, VA), you pay a late enrollment penalty when you do sign up.

The penalty: 1% of the national base beneficiary premium for each month you were eligible but not enrolled. Multiplied by your number of uncovered months.

The 2026 national base beneficiary premium is approximately $36.78/month.

Example: You delay 24 months. Penalty: 24 × 1% × $36.78 = $8.83/month, added permanently to your Part D premium.

Over 15 years: $8.83 × 12 × 15 = $1,589.

Not catastrophic. But it compounds: as the base beneficiary premium increases over time, so does your penalty calculation. And it's permanent — there's no paying it off.

Who gets caught: Healthy 65-year-olds who don't take any prescriptions. "I don't take any medications, why do I need drug coverage?" Because when you do need it — and most people eventually do — you'll pay the penalty on top of the premium. Enroll in a low-cost Part D plan even if you take no medications. They cost $10-20/month. The penalty you'll pay later for skipping is often comparable, and you lose coverage in the meantime.

Mistake #3: The COBRA Trap

This is the one that creates coverage gaps and unexpected hospital bills.

The trap: Your employer-provided health insurance ends at 65. You elect COBRA to continue that coverage while you figure out Medicare. You believe you're covered. You delay Medicare enrollment.

What actually happens: COBRA does not qualify as employer group health coverage for the purpose of Medicare's Special Enrollment Period. It is an extension of your former coverage, not active employer coverage.

If you're on COBRA at 65 and delay Part B enrollment, you are missing your Initial Enrollment Period. When COBRA ends (typically after 18 months), you will not have a Special Enrollment Period — because SEP requires leaving active employer coverage, not COBRA.

You will need to enroll in Medicare during a General Enrollment Period (January 1 – March 31 each year, with coverage starting July 1). That's a potential 6-15 month wait for coverage.

Real cost: One friend had a hospital stay during her COBRA gap — 3 days inpatient, an emergency procedure, and a two-week recovery. Without Medicare, she was on COBRA, which had a $5,000 deductible. Total out-of-pocket: $14,200.

The fix: At 65, enroll in Medicare Part A and Part B during your IEP, even if you're using COBRA. You can have both simultaneously. Medicare becomes primary for your coverage, COBRA secondary. You don't "need" COBRA once you have Medicare — you can drop it and save $500-$1,500/month in COBRA premiums.

Mistake #4: Missing the Medigap Open Enrollment Window

Medigap (Medicare Supplement) policies fill the gaps that original Medicare leaves — deductibles, coinsurance, Part B excess charges. Depending on the plan, they can cap your out-of-pocket exposure at a predictable level.

The rule: When you first enroll in Part B at 65, you have a 6-month Medigap open enrollment window. During this window, insurance companies cannot deny you coverage or charge higher premiums based on your health history. You can enroll in any Medigap plan available in your state.

What happens after: Once the window closes, you have no guaranteed issue right (with limited exceptions). Insurers can require medical underwriting — reviewing your health history — and deny you coverage or charge significantly higher premiums.

What this actually means in dollars:

A 65-year-old enrolling in Medigap Plan G might pay $120-$150/month in 2026.

A 67-year-old who missed the window and now wants Medigap faces underwriting. If they have diabetes, a prior cancer diagnosis, or certain heart conditions, they may be declined entirely. If they're accepted, the premium might be $200-$350/month — or they're offered only limited plans.

Over 20 years, the difference between a $130/month policy at 65 vs. a $280/month policy at 67 (if they can even get one) is $36,000 — before counting any coverage denials.

The secondary cost: People who miss the window and can't get Medigap often pivot to Medicare Advantage instead. Medicare Advantage has lower premiums but network restrictions, prior authorization requirements, and higher out-of-pocket maximums. For people who live in metro areas with large provider networks, this may be fine. For those with specific specialists or rural providers, the network limitations can be significant.

See Healthcare Costs in Retirement for the full breakdown of what Medicare doesn't cover.

Mistake #5: Mis-Timing the Transition from Employer Coverage

The most confusing scenario: you're still working at 65, you have good employer health insurance, and you're not sure whether to enroll in Medicare.

The general rule: If your employer has 20+ employees, your employer plan is primary coverage. Medicare, if enrolled, is secondary. In this case, it may make sense to delay Medicare Part B (you can do so without penalty under the active employer coverage exception). Part A is usually free and harmless to enroll.

The 20-employee threshold: If your employer has fewer than 20 employees, Medicare becomes primary — even if you're still working and have employer coverage. In small employer situations, you must enroll in Medicare at 65 or face potential coverage gaps.

The spouse's employer coverage trap: Many 65-year-olds skip Medicare because they're covered on their 52-year-old spouse's employer plan. That can work — but only if the spouse's employer has 20+ employees, the plan qualifies as creditable coverage (for Part D purposes), and the spouse doesn't change employers or lose coverage.

When the spouse does change employers or lose coverage, you need to enroll in Medicare within 8 months (SEP window). Miss it — even by a few months — and you're in the penalty zone.

What the transition should look like:

1. Confirm your employer's size (20-employee threshold) 2. Enroll in Part A at 65 (free, no reason to delay) 3. If employer has 20+ employees and you're actively employed: delay Part B, document your creditable coverage 4. When employment ends: enroll in Part B within 8 months 5. Start Medigap enrollment immediately — your guaranteed-issue window opens when you enroll in Part B

Premium Comparison: Original Medicare vs. Medicare Advantage

Many people at 65 face this choice without enough information.

| Factor | Original Medicare + Medigap | Medicare Advantage | |--------|----------------------------|--------------------| | Monthly premium | $185 (Part B) + $120-200 (Medigap) + $10-50 (Part D) = $315-435/month | $0-80/month (bundled) | | Out-of-pocket maximum | Usually $0-$240 (Medigap covers most gaps) | $4,000-$8,850/year | | Network | Any provider who accepts Medicare (nationwide) | Network-restricted (HMO/PPO) | | Referrals | Not required | Often required for specialists | | Prior authorization | Not required for most services | Required for many procedures |

The math favors original Medicare + Medigap for people with ongoing health needs, multiple specialists, or who want predictable costs. Medicare Advantage can work well for healthy people in areas with large provider networks.

The critical point: Medicare Advantage is easy to switch out of, but switching into original Medicare + Medigap later is hard (Medigap underwriting applies after your initial open enrollment window). If you start with Medicare Advantage at 65 and want to switch to Medigap at 70 due to health issues, you may not be able to get Medigap coverage at all.

What To Actually Do

1. Set a calendar reminder 3 months before your 65th birthday. That's when your IEP opens.

2. Enroll in Part A immediately at 65. It's free. No reason to delay.

3. Assess your employment situation. Are you actively employed with a 20+ employee firm? If yes, you can delay Part B without penalty — but get documentation. If no, enroll in Part B during your IEP.

4. Avoid the COBRA assumption. If your coverage is COBRA, not active employment, your IEP clock is ticking.

5. Enroll in a Part D plan at 65 even if you take no medications. The cheapest available plan is usually $10-20/month. The penalty for not having it is comparable and permanent.

6. Decide Medigap vs. Medicare Advantage during your 6-month guaranteed issue window. This window doesn't reopen. If you choose Medicare Advantage initially, understand that switching to Medigap later requires medical underwriting.

7. If you're on your spouse's coverage, check the employer size. Under 20 employees = you need Medicare primary.

See Your Retirement Budget Is Wrong for how healthcare costs fit into the three-phase spending model. See Estate Planning After 65 for the full financial protection checklist.

Medicare rules are complex and change annually. This article is based on 2026 guidelines. Confirm current thresholds and enrollment windows at medicare.gov. This is educational content, not legal or financial advice.