1. Assuming Social Security enrolled you
If you were already drawing Social Security at 65, Parts A and B started automatically. If you weren't, nothing happened — and people discover this months later, having been uninsured the whole time.
There is no notification for the second case. Check your status at [medicare.gov](https://www.medicare.gov) rather than assuming, and if nothing is on file, apply at ssa.gov/medicare or call 1-800-772-1213.
2. Comparing premiums instead of total cost
A $0-premium plan that puts one of your drugs on a high tier can cost more over a year than a $40-a-month plan that covers it well. The premium is the number that's easy to compare, which is exactly why it's the wrong one to decide on.
Enter your actual prescriptions in [Medicare's Plan Finder](https://www.medicare.gov/plan-compare) and sort by estimated total yearly cost. It takes five extra minutes and it is the entire point.
3. Trusting the provider directory
Online directories carry stale entries. Call the doctor's billing office and ask whether they are in network for the exact plan name for the coming plan year — not whether they "take Medicare," and not by the insurance company's name.
Do it for the hospital too, not just the primary-care doctor. See checking your own doctors and drugs.
4. Skipping Part D because you take nothing
The Part D late enrollment penalty is 1% of the national base premium for every month without creditable coverage, added to your premium permanently. At the 2026 base of $38.99 a month, a year of delay costs roughly $4.70 a month for life.
A cheap plan now almost always beats the penalty later. The exception is if you have creditable coverage elsewhere — keep the letter that says so.
5. Letting the Medigap window close
Your six-month Medigap open enrollment starts the month you are both 65 and enrolled in Part B. During it, no insurer can decline you or charge more for your health history. After it, in Wyoming and Utah, they can — and a diagnosis in the meantime can make Medigap unavailable at any price.
This is the one Medicare deadline you cannot get back. See the Medigap 6-month window.
6. Relying on COBRA past 65
COBRA is not coverage based on active employment, so it does not let you delay Part B without penalty. People carry COBRA for months believing they're protected and emerge with a permanent Part B penalty and a gap in coverage.
Active employer coverage does extend your window. COBRA and retiree coverage generally do not.
7. Not paying the first premium
Especially on Marketplace plans. Enrolling does not start coverage; the first payment does, and the deadline often falls before the coverage start date. If no bill has arrived within two weeks, call the insurer — a missing bill does not remove the deadline.
Then verify independently: how to confirm your coverage actually started.
8. Auto-renewing without looking
Do nothing during the annual window and you're usually rolled into the same plan — but the premium, the formulary, the network and, for Marketplace coverage, your subsidy may all have changed. Every plan mails an Annual Notice of Change each fall precisely because things changed.
Re-run your drug list every year. Medicare's window is October 15 to December 7; the Marketplace runs November 1 to January 15. It's an hour, once a year, and it is the single highest-value hour in this article.
None of this requires an agent
Every mistake above is avoidable with free tools and a phone. That's the honest position: self-enrollment works, and the people who get burned are almost always the ones who skipped a check, not the ones who lacked an agent.
Where a second pair of eyes genuinely helps is a long prescription list, Medigap underwriting, or employer coverage timing. Our honest look at whether you need an agent sets out both sides, and everything else lives on the self-enrollment hub.
Frequently Asked Questions
What's the most common self-enrollment mistake?
Comparing monthly premiums instead of estimated total yearly cost with actual prescriptions entered. It's the easiest number to compare and the wrong one to decide on.
Does COBRA let me delay Part B without a penalty?
No. Only coverage based on active employment does. Relying on COBRA past 65 commonly produces a permanent Part B late enrollment penalty and a coverage gap.
What happens if I do nothing during open enrollment?
You're usually renewed into the same plan, but its premium, drug list, network and your subsidy may all have changed. The Annual Notice of Change your plan mails each fall lists what's different.
Is enrolling myself riskier than using an agent?
Not inherently. Every mistake here is avoidable with free government tools and a couple of phone calls. An agent adds most value on long drug lists, Medigap underwriting, and employer-coverage timing.
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