Seven Open Enrollment Mistakes That Cost You All Year

Open enrollment is a short window in which you make decisions that then bind you for twelve months. Almost every error in it is irreversible until the following year, and almost every error is made by doing nothing rather than by choosing wrongly.

What follows are the seven that cost the most, with the figures they turn on. The first is by far the most common.

1. Letting last year’s plan roll over

Most employer plans re-enrol you automatically in whatever you had. That feels like continuity and is not, because the plan itself has changed underneath you. Premiums, deductibles, coinsurance, the network of doctors and the drug formulary are all reset annually, and none of those changes require your agreement.

The statutory ceiling has moved too, which tells you which direction plan design is travelling. The Centers for Medicare and Medicaid Services set the maximum annual limitation on cost sharing at 10,600 dollars for self-only coverage and 21,200 dollars for other than self-only coverage in 2026. For 2027 those figures rise to 12,000 dollars and 24,000 dollars, which CMS describes as an increase of approximately 13.2 percent.

That is the legal ceiling, not what your plan charges. It is worth knowing because it is the outer boundary of what a compliant plan is allowed to expose you to, and it is moving upward faster than wages.

The fix is twenty minutes with the summary of benefits and coverage for the plan you already have, looking specifically at what changed.

2. Choosing on the premium and ignoring the ceiling

The premium is the only number you see monthly, which is why it dominates the decision and why the decision is often wrong. The figure that determines how bad a bad year gets is the out-of-pocket maximum.

The way to compare plans properly is to price three scenarios, not one. Here is that comparison for two illustrative plans: a high-deductible plan at 120 dollars a month with a 3,400 dollar deductible, 20 percent coinsurance, a 7,000 dollar out-of-pocket maximum and a 750 dollar employer health savings account contribution; and a lower-deductible plan at 320 dollars a month with a 1,000 dollar deductible, 20 percent coinsurance and a 4,000 dollar out-of-pocket maximum.

Total cost here is a full year of premiums, plus what you pay toward care, less the employer account contribution.

The point is not that the high-deductible plan wins. It is that you cannot know which wins without running the three rows, and that the worst case, the row people assume settles the argument, is the row where these two plans are almost identical. Change the premium gap by 50 dollars a month and the ranking inverts. Run it with your own plan documents.

3. Skipping the network and formulary check

Networks and drug formularies are revised every year, and the revisions are not announced in a way you will notice.

Two checks, both worth doing before you choose rather than in February. Look up each doctor, specialist and hospital your household actually uses in the new plan’s directory, by name, for the coming plan year. Then look up each prescription in the new formulary, noting the tier, because a drug moving from one tier to another can change what you pay several-fold, and a drug leaving the formulary can change it entirely.

Prior authorisation requirements also change. If a medication or a recurring treatment currently needs one, confirm whether that continues, because a gap in authorisation is the thing that interrupts treatment in January.

4. Getting the flexible spending account election wrong

A health FSA is funded by a salary reduction you elect now and generally cannot change later. The IRS set the 2026 limit in Revenue Procedure 2025-32: the dollar limitation on voluntary employee salary reductions for contributions to health flexible spending arrangements is 3,400 dollars, and the maximum carryover amount is 680 dollars.

Two mistakes follow from that pair of numbers. Electing too much leaves you scrambling in December, because anything above the 680 dollar carryover is forfeited if your plan offers a carryover at all; some offer a grace period instead, and some offer neither. Electing too little wastes the tax break on spending you were going to do anyway.

The method that works is arithmetic rather than instinct. Add up last year’s actual out-of-pocket spending, subtract anything genuinely non-recurring, add anything you already know is coming, such as a planned procedure or new glasses, then elect slightly under the result. Under is the right side to err on, because the downside of a small shortfall is paying with after-tax money and the downside of an overshoot is forfeiting the money entirely.

5. Combining accounts that cannot be combined

This one is technical and expensive. A general purpose health FSA disqualifies you from contributing to a health savings account, and so does your spouse’s general purpose FSA, because it can reimburse your expenses.

So a household that elects a high-deductible plan in order to use an HSA, while one partner quietly re-enrols in a general purpose FSA at their own employer, has cancelled the HSA eligibility without anybody noticing until the tax return. The compatible alternative is a limited purpose FSA, restricted to dental and vision, which most employers offering high-deductible plans also offer.

The related omission is simpler: choosing the high-deductible plan and then never opening the HSA. The plan’s arithmetic in the table above depends on the employer contribution and the tax treatment. Without the account, you have taken the deductible and left the compensation.

6. Missing a deadline, on the wrong calendar

There are three separate enrollment calendars and they do not align.

HealthCare.gov states that for Marketplace coverage you must enrol in or change plans by 15 December and pay the first premium for coverage beginning 1 January, with 15 January the last day to enrol at all. Medicare’s own guidance sets its open enrollment at 15 October to 7 December, with the plan required to receive the request by 7 December.

Outside these windows you need a qualifying life event. HealthCare.gov allows a special enrollment period if you lost qualifying health coverage in the past 60 days or expect to lose coverage in the next 60 days, and lists marriage, a birth, adoption or foster placement, divorce with loss of coverage, a death in the household, a move, and turning 26 among the qualifying events. Note that you may apply up to 60 days before a known loss, which is the version almost nobody uses and the version that avoids a gap.

7. Treating it as a medical decision only

Open enrollment is usually the one moment each year when the whole benefits package is open, and the non-medical elections are the ones that get skipped because they are not urgent.

Check the beneficiary designations on life insurance and the retirement plan, which override your will and are frequently years out of date. Check whether the employer life insurance is adequate and whether any supplemental coverage you are paying for is worth its price. Look at disability coverage, which is the benefit most people are most underinsured for and least likely to review. Confirm dependants are correctly listed, particularly after a birth, a marriage or a child ageing off the plan. And if dependent care or commuter benefits are on offer, decide deliberately rather than by omission.

The one document to read

Plan brochures are marketing. The document that is designed to be compared is the Summary of Benefits and Coverage, which HealthCare.gov describes as an easy-to-read summary that lets you make apples-to-apples comparisons of costs and coverage between health plans. Every plan must produce one, and they follow the same layout, which is the entire point.

Four things to extract from each one, in this order: the deductible, the out-of-pocket maximum, the coinsurance percentage after the deductible, and whether the deductible is separate for prescriptions. That fourth item catches people, because a plan can advertise a modest medical deductible while running a separate pharmacy deductible that a household on regular medication will hit first.

The SBC also includes standardised coverage examples showing what the plan would pay in specific scenarios. Those examples are not your circumstances, but they are calculated identically across plans, which makes them a fair basis for ranking.

If you missed the window

For employer coverage, a missed deadline generally means the default election stands until the next plan year, unless you have a qualifying life event. It is still worth asking benefits, since some employers allow corrections for a short period and none of them advertise it.

For Marketplace coverage, the special enrollment rules above are the route back in, and the 60-day clock runs from the event. Keep documentation of the date, because you will be asked to prove it.

For Medicare, missing the 15 October to 7 December window means the plan you have continues into the new year. Anyone already in a Medicare Advantage plan has a second chance between 1 January and 31 March to switch plans or return to Original Medicare, but that window does not help someone in Original Medicare who wanted to change drug coverage.

In every case, the useful response is the same: diarise next year’s window now, while the cost of missing it is fresh.

The order to work through it

1. Find the deadline for your own plan and put it in the calendar with a week of margin.
2. Pull last year’s actual spending: premiums, deductibles, copays, prescriptions, everything.
3. Build the three-scenario comparison above for each plan offered, using the real figures from the summary of benefits and coverage.
4. Check every doctor and every prescription against the new network and formulary.
5. Set the FSA election from the spending history, erring low, and confirm whether your plan offers a carryover or a grace period.
6. If you choose a high-deductible plan, open and fund the HSA, and make sure no general purpose FSA exists in the household.
7. Review beneficiaries, disability, life and dependants before you submit.

What this cannot do

Careful open enrollment moves money at the margin. It does not change the fact that the plans on offer are chosen by your employer, that the cost-sharing ceiling is rising at 13.2 percent into 2027, and that the best available choice on a bad menu is still a bad choice.

The realistic benefit is narrower and still worth having: you avoid the avoidable errors, you know what a bad year would cost before it happens, and you stop paying for a plan that stopped matching your circumstances two jobs ago. That is not the same as coming out ahead, and it is worth being honest about the difference.

General information, current as of September 2026, and not personalised insurance, tax or medical advice. Plan terms, limits and deadlines change annually; check your own plan documents and the primary sources above.