Small enrollment mistakes can cost thousands of dollars over the years. This page lists the most common ones. Each includes what people do wrong, why it hurts them, and how to avoid it. Use this as a checklist before you enroll, and again every year during open enrollment.
Missing Your Enrollment Window
What People Do
They assume they can sign up for Medicare anytime, or they miss the deadline by a few days because they're busy, wait for mail that doesn't come, or think a phone agent was wrong about the date.
Why It Costs
Missing your 7-month Initial Enrollment Period for Medicare means a permanent late penalty on your Part B premium (usually 10% per year, forever). You also get a coverage gap. If you had a heart attack the day your coverage should have started, you'd pay the full bill.
How to Avoid It
Put your 7-month window on a physical calendar months in advance. Set phone reminders. Create your SSA.gov and Medicare.gov accounts now. Don't trust that mail will come on time. Call 1-800-MEDICARE (1-800-633-4227) to confirm your enrollment window dates.
Assuming Employer or Retiree Coverage Lets You Delay Part B Without Penalty
What People Do
They think any health coverage — employer, retiree, COBRA, VA — lets them delay Part B without getting a late penalty.
Why It Costs
For Part B, only coverage from your (or your spouse's) current job lets you delay safely — and for most people the employer needs 20 or more employees. Retiree benefits, COBRA, and VA health care do NOT protect you from the Part B penalty. ("Creditable coverage" is a different test — it's about drug coverage and the Part D penalty, not Part B.) Delay on the wrong basis and you're stuck with a lifelong penalty on your Part B premium.
How to Avoid It
Ask your employer's HR or benefits office two separate questions, and get the answers in writing: "Does my coverage count as current-employer coverage so I can delay Part B?" and "Is my drug coverage creditable for Part D?" For VA coverage, ask your VA benefits counselor. See Working Past 65 and Retiring or Losing Coverage for your specific situation. When in doubt, call your SHIP counselor to review before you delay.
Forgetting the Medigap Open-Enrollment Window
What People Do
If they choose Original Medicare, they don't realize they have only 6 months (their Medigap Open Enrollment Period) to buy a Medigap supplement without answering medical questions. They delay past that window, then can't get a supplement or have to pass medical underwriting and might be denied.
Why It Costs
After your 6-month window closes, insurers can ask about your health and deny you or charge more if you have health problems. You could lose the freedom to switch doctors or specialists. Or you pay higher out-of-pocket costs in Original Medicare without a Medigap backup.
How to Avoid It
If you choose Original Medicare, buy a Medigap plan within 6 months of turning 65 AND enrolling in Part B. See Medigap Plans for your options. Act early — don't wait until month 5. This window closes and doesn't reopen unless you have a special life event.
Choosing Medicare Advantage Without Understanding the Lock-In
What People Do
They choose a Medicare Advantage plan without realizing that if they later want to switch to Original Medicare and buy a Medigap supplement, they might face medical underwriting or be denied outright.
Why It Costs
Medicare Advantage is a one-way door. You can switch Advantage plans every year, but getting back to Original Medicare + Medigap is hard. If you have new health problems after choosing Advantage, you might not qualify for a Medigap supplement. You get trapped in Advantage or on Original Medicare without supplement coverage.
How to Avoid It
Before choosing Advantage, understand this lock-in. If you think you might want to go back to Original Medicare later, be cautious. Read Choosing Coverage to weigh Original vs. Advantage carefully. If you do choose Advantage, review it every year and consider switching back during open enrollment if your health or situation changes.
Contributing to an HSA After Part A Enrollment
What People Do
They keep contributing to a Health Savings Account after they enroll in Medicare Part A, unaware that it's not allowed and creates tax complications.
Why It Costs
Once you enroll in Part A, you can't contribute to an HSA anymore — it's against the rules. If you do, the IRS treats it as a mistake and you might owe taxes and penalties. Some people don't realize until they file taxes the next year.
How to Avoid It
Plan ahead — stopping the month you enroll can already be too late. If you sign up for Part A after your first enrollment window, Part A can start retroactively, up to 6 months back. That means you generally need to stop HSA contributions 6 months before you apply (or before you file for Social Security, which triggers Part A automatically). Tell your employer's benefits office. See Working Past 65 for details on this trap and how to handle it.
Not Reviewing Your Plan Every Year
What People Do
They stay in the same Medicare Advantage or Part D plan year after year without checking if it still works for them. Networks change, formularies change, and premiums go up.
Why It Costs
A plan that was perfect last year might cover your doctor differently this year. A drug that was on tier 2 might move to tier 4. Your premium might jump $50 a month. A cheaper plan might be available. You miss the chance to switch during open enrollment and overpay for a year.
How to Avoid It
Every October, during open enrollment, go back to Medicare.gov Plan Finder or call your SHIP. Spend 30 minutes comparing plans. Check whether your doctor is still in-network. Confirm your drugs are still on the same tiers. Switch if you can save money or get better coverage. This is free and takes less time than one doctor's appointment.
Thinking Plan Finder's Doctor List Is Complete
What People Do
They see their doctor listed as in-network on Medicare Plan Finder and enroll thinking they're set. Then they call the doctor to schedule an appointment and learn the doctor left the plan, or isn't taking new patients.
Why It Costs
Out-of-network doctors cost a lot more. You might end up switching doctors, losing continuity of care, or paying thousands of dollars in surprise bills. Plans' network information isn't always current.
How to Avoid It
Plan Finder is a starting point, not the final word. After you narrow to a plan, call your doctor's office and ask: "Are you in network with [Plan Name]? Are you taking new patients?" Then call the plan and confirm the same thing. See How to Use Medicare.gov Plan Finder for the full verification process.
Ignoring Renewal Notices from Medicaid
What People Do
Medicaid sends renewal paperwork or asks them to update information online. They miss the deadline, forget to respond, or assume they're already approved and don't need to do anything.
Why It Costs
If you don't respond by the deadline, Medicaid drops you. You lose coverage even if you still qualify. It can take weeks to reapply and get reinstated. In the meantime, a doctor visit, prescription, or hospitalization can cost thousands.
How to Avoid It
Don't ignore Medicaid mail. Open it, read the deadline, and respond. If your state has online renewal, do it immediately. Keep Medicaid updated on address changes so you don't miss mail. Mark your calendar for your renewal date.
Not Checking if You Qualify for Help Paying
What People Do
They struggle with premiums and drug costs without realizing free programs exist. They think they make "too much" money, or don't think they qualify because they have Social Security.
Why It Costs
Medicare Savings Programs, Extra Help, and Medicaid can pay your premiums and reduce drug costs to $0 or $5. Many people leave thousands of dollars on the table because they didn't apply. Income limits are often higher than people think.
How to Avoid It
If money is tight, assume you might qualify. Go to Healthcare.gov, call 1-800-MEDICARE, or visit your state Medicaid office. Ask about programs that help. It takes 15 minutes to apply. See Getting Help Paying for the full list.
Not Comparing Part D Plans During Open Enrollment
What People Do
They stay in the same Part D plan or assume the cheapest one is best without looking at their specific drugs. They compare premiums instead of total estimated cost.
Why It Costs
Two Part D plans with the same $35 premium can have very different drug costs. One might charge $5 for your main drug, the other $50. Over a year, that's a $540 difference. Cheaper premiums don't mean cheaper coverage for you.
How to Avoid It
Use Plan Finder and enter all your drugs. Compare total estimated cost, not just premium. Call your SHIP if you need help. See How to Use Medicare.gov Plan Finder for step-by-step guidance.
Going It Alone Instead of Calling Free Counselors
What People Do
They spend hours researching Medicare alone. They feel embarrassed asking for help. They don't realize free, expert, unbiased help exists.
Why It Costs
Without expert guidance, people make enrollment mistakes, stay in expensive plans, miss deadlines, or skip programs they qualify for. These mistakes cost thousands over the years.
How to Avoid It
Call your SHIP (State Health Insurance Assistance Program) at shiphelp.org. Or call 1-800-MEDICARE (1-800-633-4227). These counselors are free, unbiased, and trained. They've answered these questions a thousand times. It's the best use of 30 minutes you can make.