5 Medicare Enrollment Errors That Reduce Coverage

Signing up for Medicare feels like it should be a milestone to celebrate, but for many new retirees, it is a minefield of irreversible decisions. Unlike private insurance, where you can usually fix a mistake the following year, Medicare has strict “lock-in” rules. If you miss a specific window or misunderstand how a secondary plan works, you may be saddled with lifetime financial penalties or permanently barred from the coverage you want.

In 2026, the stakes are higher than ever. With stricter IRS auditing on health savings accounts and narrower provider networks, the margin for error has vanished. A simple clerical oversight during your Initial Enrollment Period can leave you with a 20% gap in coverage that no insurance company will sell you a policy to fix. Here are the five most common and costly enrollment errors older adults are making right now and how to avoid falling into the trap.

1. The “COBRA” Trap

This is the single most expensive mistake made by early retirees. If you leave your job at 65 and decide to keep your employer’s health insurance via COBRA because it covers your spouse or dental, you have walked into a trap. Medicare does not consider COBRA to be “creditable coverage” for Part B. You have an 8-month Special Enrollment Period that starts the month your employment ends, not when your COBRA ends.

If you wait 18 months until your COBRA runs out to sign up for Part B, you will miss your window. You will face a permanent Late Enrollment Penalty (10% for every year missed) and, worse, you will be forced to wait until the next General Enrollment Period to sign up, leaving you potentially uninsured for months.

2. The HSA “Tax Bomb”

If you plan to work past 65 and keep contributing to your Health Savings Account (HSA), you must be extremely careful. The IRS prohibits you from contributing to an HSA once you are enrolled in any part of Medicare (Part A or Part B). The error happens when people apply for Social Security benefits at 66 but don’t realize that doing so automatically enrolls them in Medicare Part A.

Furthermore, Part A coverage is retroactive up to six months. If you or your employer contributed to your HSA during those six “retroactive” months, you have made an illegal “excess contribution.” You will be hit with tax penalties and interest unless you reverse those contributions before tax day. To avoid this, you must stop all HSA contributions six months before you apply for Medicare.

3. Missing the Medigap “Golden Ticket”

When you first turn 65 and enroll in Part B, you are handed a one-time “Golden Ticket”: a 6-month Medigap Open Enrollment Period. During this window, you can buy any Medigap (Supplement) policy on the market, regardless of your health. Insurance companies cannot deny you or charge you more for pre-existing conditions. The error occurs when healthy seniors decide to save money by choosing a cheap Medicare Advantage plan initially, thinking, “I’ll switch to a Medigap plan later when I get sick.” This is often impossible.

Once that 6-month window closes, it is gone forever in most states. If you try to switch to Medigap three years later because you developed cancer, you will be subject to Medical Underwriting. The insurer can (and likely will) deny your application based on your health history, leaving you stuck in your Advantage plan with its restricted network.

4. The “I Don’t Take Meds” Part D Skip

Healthy seniors often look at the monthly premium for a Part D drug plan and decide to skip it because they take no prescription medications. This saves money in the short term but costs a fortune in the long run. Medicare calculates a Part D Late Enrollment Penalty of 1% of the national base beneficiary premium for every month you went without coverage.

If you stay healthy for five years and then develop a heart condition requiring expensive meds, you will be forced to pay a penalty (roughly 60% on top of your premium) for the rest of your life. Even if you take zero pills, you should enroll in the absolute cheapest Part D plan available (often under $10/month) simply to serve as “insurance for your insurance” and avoid the penalty clock.

5. The “Auto-Pilot” Renewal

Assuming your plan will stay the same next year is a dangerous complacency. Medicare Advantage and Part D plans change their contracts every January 1st. They can drop your doctor, remove your drugs from the formulary, or increase your maximum out-of-pocket limit.

The error is letting the plan “auto-renew” without checking the Annual Notice of Change (ANOC). If your cardiologist leaves the network in 2026 and you don’t switch plans during the Open Enrollment Period (Oct 15 – Dec 7), you will be stuck paying out-of-network rates to see them for the entire year. You must actively audit your plan every single autumn, no matter how much you liked it this year.

Deadlines Are Not Suggestions

In the world of Medicare, a deadline is a wall, not a hurdle. The government is unforgiving of “I didn’t know” excuses. The only way to protect your retirement savings is to treat your enrollment dates with the same seriousness as your tax returns. Mark your 65th birthday, your retirement date, and every October 15th on your calendar. If you are unsure about a rule—especially regarding COBRA or HSAs—consult a licensed State Health Insurance Assistance Program (SHIP) counselor before you sign (or don’t sign) anything.

Did you accidentally trigger a penalty because of the HSA 6-month rule? Leave a comment below—your experience could save another reader from a tax nightmare!