The 401(k) Contribution Mistake Many Savers Do Not Notice Until Tax Season

The tax surprise usually is not that you saved too little. It is that your payroll election sent money into a different 401(k) tax bucket than you thought. Traditional 401(k) salary deferrals generally are not included in box 1 wages for federal income tax purposes, while designated Roth 401(k) contributions are included in box 1 and reported separately in box 12. (irs.gov)

That matters because the account balance can grow either way, so the mistake can hide in plain sight until your W-2, your withholding, or a smaller-than-expected refund exposes it. The IRS also says pre-tax and Roth employee deferrals share one annual limit, so this is not just a tax issue. It can also turn into a contribution-limit problem when you split contributions or change jobs midyear. (irs.gov)

The real mistake: treating every 401(k) dollar the same

Many workers choose a contribution percentage and stop there. But a 10% election into a traditional 401(k) is not the same as a 10% election into a Roth 401(k), and neither is the same as plan-level after-tax contributions. The IRS explains that designated Roth contributions are included in income when contributed, unlike pre-tax salary deferrals. (irs.gov)

There aren’t many arguments stating Roth is a bad option. Roth could provide you with a wise investment. The issue lies with accidental taxation classification. Many people assumed Roth gave them a current-year benefit (or assumed they were building Roth savings) and then discovered when they received their W-2, that they had been incorrectly classified.

The other reason this mistake is easy to miss is that a correct traditional election still does not reduce every wage box. The IRS says elective deferrals generally are not current federal income-tax wages, but they are still included as wages for Social Security and Medicare taxes. That is why a traditional 401(k) can be working properly even if boxes 3 and 5 on the W-2 stay higher than you expected. (irs.gov)

Use the TAX Audit before your last paycheck, not after your W-2

Most of these issues will be discovered through a standard audit. The tools used by this audit are TAX: Tax bucket, Annual cap and eXit test. The auditor will check for three things before it becomes tax time to make sure they have confirmed 1) Where did my money go, 2) Why I am still under my annual limit and, 3) Whether my W-2(s) are backed up by what my payroll department said occurred.

1. Tax bucket: On your latest paystub or benefits portal, look for separate labels for traditional 401(k), Roth 401(k), and after-tax contributions. If your goal is a current-year federal tax break, only traditional or pre-tax deferrals generally do that. (irs.gov)
2. Annual cap: Add all of your employee elective deferrals across every employer for the calendar year. For 2026, the regular limit is $24,500. If you are age 50 or older, the catch-up is $8,000; if you are age 60, 61, 62, or 63 in 2026, the higher catch-up is $11,250 if your plan permits it. Plans may impose lower limits. (irs.gov)
3. eXit test: When the W-2 arrives, compare your year-end totals with box 12. Traditional 401(k) deferrals generally show as code D and usually are not included in box 1 wages. Roth 401(k) contributions generally show as code AA and are included in box 1. (irs.gov)
4. If you changed jobs: Reduce the second employer election by what you already contributed at the first employer. The employee limit follows you across employers. (irs.gov)
5. If anything does not match: Ask payroll or the plan administrator for a written explanation before you file. The SPD and individual benefit statements are supposed to explain plan contributions and show account details. (dol.gov)

A realistic example with numbers

Consider Erin, age 38, earning $110,000. She contributes 10% all year, or $11,000, and assumes her taxable wages will drop by about that amount because she thinks she picked pre-tax. If the election really is traditional, her W-2 box 1 could land around $99,000 before any other payroll items. If payroll actually sent the money to a Roth 401(k), box 1 could stay near $110,000 instead. The savings rate is the same, but the current-year tax result is very different. (irs.gov)

If Erin is in the 22% marginal federal bracket, that difference is roughly $2,420 of current-year federal income-tax exposure that she did not expect. The problem is not that she saved too much. It is that she assumed the tax label instead of verifying it.

Now add a job change. Erin contributes $7,500 at employer No. 1, then another $18,000 at employer No. 2. Her total employee deferrals for 2026 are $25,500, which is $1,000 over the regular 2026 limit. The IRS says the limit follows the worker across employers, and excess deferrals generally should be corrected by April 15 of the following year. For a 2026 excess, that usually means April 15, 2027. (irs.gov)

What tax season clues usually reveal the problem

• Your W-2 shows code AA, but you expected code D. (irs.gov)
• Box 1 wages are much higher than you expected after a year of contributions. Roth treatment is a common reason. (irs.gov)
• Boxes 3 and 5 did not fall even though you used traditional 401(k) contributions. That is usually normal because traditional deferrals are still generally subject to Social Security and Medicare taxes. (irs.gov)
• You contributed through two employers and never added the totals together. (irs.gov)
• Your statement mentions after-tax contributions separately from Roth contributions. (irs.gov)

Common mistakes that make the surprise worse

• Trying to claim a separate deduction on your Form 1040 for regular payroll 401(k) deferrals. Traditional deferrals generally were already excluded from box 1 wages, while Roth 401(k) contributions were already included. (irs.gov)
• Assuming Roth and traditional 401(k) limits are separate. They share one employee elective-deferral limit. (irs.gov)
• Ignoring plan-specific rules. The IRS says plan terms may impose a lower deferral limit than the tax-law maximum. (irs.gov)
• Counting employer match against your employee limit. IRS guidance describes separate tests for employee elective deferrals and overall account contributions. (irs.gov)
• Confusing after-tax contributions with Roth contributions. After-tax money can create different rollover treatment, and partial distributions generally include pretax and after-tax amounts on a pro rata basis. (irs.gov)

When changing your election will not solve it

If the issue is just future tax treatment, changing the payroll election can help going forward. But last year’s result usually does not change just because you wanted a different bucket. The tax treatment flows from the type of contribution made through payroll and how it was reported on Form W-2. (irs.gov)

If you exceeded the annual limit, this is a correction issue, not a preference issue. Contact the plan administrator quickly. IRS guidance says excess deferrals that are not returned by April 15 of the following year can be taxed once in the year deferred and again when later distributed. (irs.gov)

If payroll used the wrong code or your W-2 does not match your pay records, ask for a written correction path. And if the plan allows after-tax contributions and you are not sure what you contributed, ask for the SPD, the year-end statement, and a transaction history before moving money or rolling it over. After-tax amounts inside a plan follow their own rollover rules. (dol.gov)

How to pressure-test your setup before filing

1. Pull four documents: your final paystub, Form W-2, year-end plan statement, and your enrollment confirmation or benefits-election screen.
2. Compare your year-end traditional and Roth totals with box 12 codes D and AA. If the labels do not match, pause before filing. (irs.gov)
3. Add employee deferrals across all employers for the same calendar year, especially if you changed jobs. (irs.gov)
4. Review the SPD or any summary of material modifications if your employer changed recordkeepers, payroll systems, or plan features. DOL guidance says these documents explain contributions and plan changes, and participant-directed plans must furnish regular benefit statements. (dol.gov)
5. Keep the written response from payroll or the plan administrator with your tax records if a correction is needed.

Bottom line

The costly mistake is not choosing Roth over traditional, or traditional over Roth. Either can be reasonable. The costly mistake is assuming the label without checking it. A five-minute TAX Audit can help confirm whether your contribution type, W-2 reporting, and annual limit all line up before tax season turns a payroll setting into a tax problem. (irs.gov)

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