Updated August 6, 2026. Quick answer: the expensive mistakes in October are not choosing the wrong plan — they are not looking at all, assuming next year’s plan is this year’s plan, and believing you can undo it in January. Only one of those is even partly true.
1. Not looking, because nothing seems to have changed
Plans change every year: the drug list, the tiers, the network, the premium, the deductible. The notice explaining what changed arrives in the autumn and is the single most ignored document in American retirement. The plan you were happy with in 2026 is a different product in 2027, and it changed without asking you.
2. Checking the premium and not the drug list
A plan can drop a drug, move it to a higher tier, or add a prior-authorisation requirement without changing its premium at all. The premium is the number that is easy to compare and the one least likely to be what costs you money. Check each of your own prescriptions against next year’s formulary, at your own pharmacy. How to compare drug plans properly, and what the out-of-pocket cap does and does not cover.
3. Assuming your doctors are still in network
Medicare Advantage networks are re-negotiated annually and a practice can leave mid-year. Confirm with the practice, not only with the plan directory, which is frequently out of date. What a narrow network actually costs you and what prior authorisation adds to it.
4. Believing January will let you undo it
Partly true, and the part that is false is the expensive part. The January–March window is open only to people already in a Medicare Advantage plan, and it lets them switch plans or return to Original Medicare. It does not let someone in Original Medicare join Advantage, and it does not let them change a standalone drug plan. The two windows, side by side, from Medicare’s own table.
5. Assuming Medigap will take you back
The one that costs the most and is discovered the latest. Leaving Medicare Advantage does not give you a right to buy a Medigap policy — outside a guaranteed-issue situation, the insurer can underwrite your health and say no. If a Medigap policy is part of your plan for later, the time to understand this is before you enrol in Advantage, not after. The underwriting window, trial rights, the states with a birthday rule, and what switching actually involves.
6. Treating December 7 as a postmark date
It is not. Medicare.gov states the requirement as coverage starting January 1 “(the plan must get your enrollment request by December 7)” — the plan receiving it, not you sending it. Leave a working week.
7. Forgetting that this year’s income sets a bill two years out
Nothing you do in October changes your premium surcharge for next year, because that was decided by a tax return filed two years ago. But what you do with income this year — a Roth conversion, a property sale, a large distribution — sets the surcharge for the year after next. The two-year lookback, why it is a cliff rather than a slope, and the form to use when your income has already dropped.
8. Taking advice from someone paid only when you switch
Most Medicare help is free to you because it is paid for by the plan you end up in. That is not automatically bad, and it is worth understanding before you take a recommendation: how Medicare brokers get paid. Your state SHIP programme offers genuinely free, unpaid counselling; it is the comparison to weigh a broker against.
Sources
Enrollment-period rules and the December 7 requirement are quoted from medicare.gov’s own enrollment table, read 2026-08-06. Work the season with the open-enrollment checklist and the glossary. Nothing on this page is sold, recommended, or paid for, and there are no affiliate links on it. See methodology and corrections.