8 Open Enrollment Mistakes That Quietly Cost You Money

None of these are dramatic. That is the problem with them — each one is a small omission on a benefits form in November that turns into a real number some time the following year, long after anyone connects the two.

Here they are in rough order of what they cost, with the arithmetic attached.

1. Choosing on Premium Alone — up to $2,358

The single most expensive habit, and the one the portal actively encourages by sorting plans by the smallest number on the payslip.

Three plans from the same menu, costed properly across three usage years:

The lowest-premium plan wins one year in three. In the other two it loses by $1,043 and $1,048 to a plan that costs $1,452 more in premium. And the gap between the best and worst available choice in a bad year is $2,358.

The fix takes twenty minutes: cost each plan as premium plus copays plus deductible plus coinsurance, capped at its out-of-pocket maximum, and do it three times. The full method is here.

2. Ignoring the Employer HSA or HRA Contribution — $750

Many employers seed an HSA or HRA, and almost nobody nets it off the premium when comparing.

In the example above, the high-deductible plan’s $3,068 annual premium is really $2,318 after $750 of employer money. That $750 is more than half the annual premium gap between it and the next plan — enough, on its own, to reverse a comparison.

It cuts the other way too: employer money counts against your own HSA limit. The 2027 family limit is $9,000, so a $750 seed leaves you $8,250 of room. Elect $9,000 yourself and the $750 excess is taxable and charged 6% for every year it stays in the account.

3. Over-Electing a Health FSA — whatever you don’t spend

The mistake with no remedy. The election is fixed for the plan year barring a qualifying life event, and unspent money is forfeited beyond a carryover of up to $680 for 2026 — if your employer offers a carryover at all. They may offer a grace period of up to two and a half months instead, or neither.

Elect $3,000 against $2,000 of genuine expenses and you have not saved tax on $3,000. You have handed roughly $320 back, after the carryover.

Anchor the election on what you actually spent out of pocket last year, not on an intention. And find out which of the three options your employer offers before you pick a number.

4. Missing the Window — twelve months of the wrong plan

Employer windows typically run two to four weeks in October or November. Miss it and you keep your current elections for another year — except the FSA, which drops to zero, because an FSA has to be elected again every single year.

For 2027 Marketplace coverage the federal window runs 1 November 2026 to 15 January 2027, and you must choose by 15 December 2026 for coverage starting 1 January.

Afterwards the only way in is a qualifying life event — marriage, divorce, birth or adoption, a change in employment, loss of other coverage — each opening a special enrollment period of 30 or 60 days. Put the close date in your calendar with a week’s warning, not the day itself.

5. Assuming Your Plan Is the Plan You Had

Plans get redesigned between years. Deductibles rise, copay tiers get reshuffled, out-of-pocket maximums move, formularies change, and a plan can be discontinued entirely and mapped to a “closest equivalent” that nobody asked you about.

Passive re-enrollment rolls you into something. It does not roll you into what you had, and it never re-elects your FSA.

Pull the new Summary of Benefits and Coverage and compare it against last year’s, field by field. It is a five-minute job and it is the only way to find out that your deductible went up $500.

6. Not Running Your Prescriptions Against Each Formulary

A drug that is tier 1 on one plan and non-formulary on another can move a household’s annual cost by more than the entire premium difference between the plans — and unlike a deductible, there is no cap protecting you, because a non-covered drug does not count toward the out-of-pocket maximum at all.

List every regular prescription. Check each one against each plan’s formulary, including the tier, before you compare anything. Do the same for must-keep doctors: an HMO may have no out-of-network coverage whatsoever, so a specialist you rely on being outside the network is a hard constraint rather than an extra cost.

7. Funding an HSA and a General-Purpose Health FSA in the Same Year

Not allowed, and the portal will usually let you do it without a word.

A general-purpose Health FSA counts as first-dollar health coverage, which makes you ineligible to contribute to an HSA. Two ways this catches people:

• A spouse’s Health FSA disqualifies you, even on separate plans, even with no access to the account.
• A grace period extends the problem into the next year — an FSA with a two-and-a-half-month grace period can keep you HSA-ineligible until mid-March, making January and February contributions excess.

The fix is a limited-purpose FSA, dental and vision only, which pairs with an HSA without issue. The full comparison of the two accounts covers where each one earns its place.

8. Prorating a Part-Year HSA in the Wrong Order — 6% a year, forever

If you become HSA-eligible mid-year, the IRS prorates the annual limit by your months of eligibility, and employer money comes off that prorated figure.

Six months of family eligibility in 2027:

The difference is $375 of excess contribution — taxable, and charged 6% every year until it is corrected. Getting the election right is four numbers and five minutes.

The Things Worth Doing in the Same Sitting

While the documents are out:

• Check the family deductible structure. Embedded individual deductibles inside a family one behave very differently from aggregate ones when a single household member has most of the costs.
• Price the ancillary benefits against what you would actually claim. Dental at $30 a month against one cleaning and a filling is not obviously worth it; disability cover usually is, and is the benefit people most often skip.
• Confirm your dependents still qualify and that one family plan really beats splitting people across two employers’ plans. It frequently does not.
• Write down what you chose and why — one paragraph, kept somewhere you will find it. When next year’s actual spending comes in, that note tells you immediately whether the estimate was any good, and next October becomes twenty minutes instead of a weekend.

None of this is hard. All of it is a thing you do once a year, which is exactly why it never gets a system — and why the same small mistakes cost the same households the same money every November.

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Most of the list above is a check the file runs for you. Plan Comparison nets employer HSA and HRA money off the premium automatically and tests each plan’s deductible and out-of-pocket maximum against the IRS rules. True Annual Cost produces the three-scenario table at the top of this page. HSA Planner subtracts employer contributions from your limit, prorates part-year eligibility in the correct order, and tells you in dollars if an election is over. FSA Planner warns on the HSA conflict and on a Dependent Care election larger than the care you expect to pay for.

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