7 Tax Mistakes That Could Cost You Money

Every year, millions of Americans leave money on the table — or hand extra cash to the IRS — not because they cheated, but because they made mistakes. Missed deadlines, wrong deductions, forgotten credits, and rookie investment errors add up fast. The good news? Most of these mistakes are completely avoidable once you know what to look for.

The tax code changes constantly. New deductions appear, old ones expire, and thresholds shift with inflation. In 2025 alone, the One Big Beautiful Bill Act introduced a flurry of new breaks — a car loan interest deduction, overtime and tip exclusions, and an expanded senior bonus — while phasing out others. Filing without checking what changed is how taxpayers end up overpaying by hundreds, sometimes thousands, of dollars.

This guide covers the seven most costly tax mistakes filers make, from the basics of deadlines and penalties to the strategies that separate smart, year-round planners from April scramble artists. Whether you're a W-2 employee, a freelancer juggling quarterly payments, or a retiree navigating required minimum distributions, at least a few of these will hit close to home.

None of this is a substitute for professional advice on your specific situation. But understanding where mistakes happen — and why — is the first step toward keeping more of what you earn. Read on, and take notes.

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Self-Employment & Freelance Taxes

Self-employment is a tax surprise that hits hardest in year one. When you work for an employer, they withhold federal income tax, Social Security, and Medicare from every paycheck. When you're self-employed, nobody does that for you. The 15.3% self-employment tax — covering both the employer and employee halves of Social Security and Medicare — is on top of ordinary income tax, and it applies to every dollar of net earnings above $400.

The IRS expects you to pay as you earn, not all at once in April. That means quarterly estimated payments — due in April, June, September, and January. Miss them and you'll owe an underpayment penalty even if you settle up when you file. The fix is to set aside roughly 25 to 30% of every payment you receive and send the IRS its share four times a year using Form 1040-ES. You can also deduct half the self-employment tax on your return, along with health insurance premiums, home office expenses, business mileage at $0.70 per mile for 2025, and retirement contributions to a SEP-IRA or solo 401(k).

Who has to pay quarterly estimated taxes?

Anyone who expects to owe $1,000 or more in federal taxes after withholding and refundable credits. That includes freelancers, independent contractors, small business owners, and anyone with significant investment income or side-gig earnings not covered by employer withholding.

What happens if you skip quarterly payments as a freelancer?

You'll owe an underpayment penalty calculated for each quarter you were short, even if you pay the full year's taxes when you file in April. The penalty isn't catastrophic, but it's avoidable with basic planning and consistent quarterly deposits.

Can self-employed people deduct a home office?

Yes, if you use the space regularly and exclusively for business. The simplified method allows a $5-per-square-foot deduction up to $1,500. The regular method deducts the actual percentage of home expenses attributable to the office, which is often larger but requires more record-keeping.

Investment & Capital Gains Taxes

Investing successfully is one thing. Keeping the after-tax gains is another. Most people understand that selling an investment for a profit creates a capital gain — but fewer understand the tax rules that determine how much of that gain they keep. Holding an asset for more than one year before selling converts the gain from ordinary income (taxed up to 37%) to a long-term capital gain (taxed at 0%, 15%, or 20%, depending on income). For many middle-income filers, the long-term rate is 15%. For some, it's 0%.

Two mistakes cost investors the most money here. The first is selling positions held less than a year without considering the tax hit — short-term gains are taxed like wages, and the difference in rates can be substantial. The second is ignoring tax-loss harvesting: the strategy of selling underperforming positions to generate losses that offset gains elsewhere in the portfolio. According to Fidelity, the wash-sale rule prohibits claiming a loss if you buy back the same or a substantially identical investment within 30 days before or after the sale. Done correctly, tax-loss harvesting is one of the most effective tools for managing your annual tax liability.

What is the wash-sale rule and why does it matter?

The IRS disallows a loss deduction if you sell an investment at a loss and repurchase the same or a substantially identical security within a 61-day window (30 days before or after the sale). The disallowed loss gets added to the cost basis of the replacement shares — it doesn't disappear, but you can't use it to offset gains in the current year.

How does the net investment income tax work?

High earners may owe an additional 3.8% net investment income tax on top of the regular capital gains rate. For 2025, this surtax applies to single filers with modified adjusted gross income above $200,000 and joint filers above $250,000. It applies to interest, dividends, capital gains, and passive rental income.

Can you avoid capital gains taxes entirely on investment earnings?

If your taxable income is low enough to fall within the 0% long-term capital gains bracket — up to $48,350 for single filers and $96,700 for joint filers in 2025 — you pay no federal tax on long-term capital gains. This creates a planning opportunity, especially for early retirees living off investments before claiming Social Security or required minimum distributions.

IRS Audits & Red Flags

The odds of getting audited are low — less than 1% for most individual filers — and budget cuts have pushed that number even lower in recent years. But Kiplinger has reported that reduced IRS staffing doesn't mean audit risk disappears. The agency's automated systems use data analytics and artificial intelligence to flag returns that look statistically out of place, and those flagged returns still get examined.

The most common audit triggers include claiming deductions dramatically higher than average for your income level, reporting large Schedule C losses that offset other income, cash-heavy businesses with inconsistent income reporting, large charitable deductions without proper documentation, and unreported foreign bank accounts. If your return contains any of these, it doesn't mean you've done anything wrong — but you need documentation ready to prove it. The IRS has three years from the filing deadline to audit a standard return, six years if it believes income was underreported by more than 25%, and no time limit if fraud is suspected.

What happens when the IRS selects your return for audit?

You'll receive a notice by mail — never a phone call. Most audits are correspondence audits requesting documentation for one or two specific line items. Office and field audits are rarer and involve meeting with an IRS agent to review your records more broadly. In all cases, respond promptly and bring thorough documentation.

Can you get audited for claiming a home office deduction?

Yes — the home office deduction is a known audit trigger, particularly when claimed by filers who also report W-2 income. The space must be used regularly and exclusively for business. The IRS scrutinizes these claims heavily, especially if the deducted percentage is large relative to income.

Do you need a tax professional if you're audited?

Not necessarily, but it helps. For simple correspondence audits, most taxpayers can handle the response themselves. For office or field audits involving complex income, investments, or business deductions, a CPA, enrolled agent, or tax attorney who can represent you before the IRS is worth every dollar.

Filing & Deadlines

Missing the April 15 deadline is the single most expensive tax mistake most people make — not because the IRS is merciless, but because delay makes everything worse. The failure-to-file penalty is 5% of unpaid taxes per month, capping at 25%. The failure-to-pay penalty is 0.5% per month. When both apply simultaneously, the clock is running on two fronts at once, and daily compounding interest piles on top.

What most filers don't realize is that an extension to file is not an extension to pay. Filing Form 4868 buys you until October 15 to submit paperwork — but taxes owed are still due April 15. If you can't pay the full amount, pay what you can and file anyway. The IRS offers short-term payment plans for balances under $100,000 and long-term installment agreements for balances under $50,000. Waiting to file because you can't pay is the most common and most avoidable filing money mistake.

What happens if you miss the tax deadline entirely and owe money?

Interest accrues daily from April 15 until the balance is paid in full. According to the IRS, the current underpayment rate is 6% for the quarter beginning April 1, 2026. On top of that, failure-to-file and failure-to-pay penalties both apply until you file and pay.

Can you get penalties removed after the fact?

Yes — through first-time penalty abatement if you've filed and paid on time for the prior three years, or through reasonable-cause relief for documented hardships like serious illness, natural disasters, or circumstances outside your control.

Should you file even if you can't pay?

Absolutely. Filing on time stops the failure-to-file penalty, which is ten times more expensive per month than the failure-to-pay penalty. File what you can, pay what you can, and set up a plan for the rest.

Deductions & Credits

The standard deduction is big enough that most filers skip itemizing — and that's often the right call. But taking the standard deduction doesn't mean you've claimed everything you're entitled to. Dozens of "above-the-line" deductions reduce your adjusted gross income regardless of whether you itemize, and millions of filers miss them every year.

Student loan interest, for example, is deductible up to $2,500 regardless of whether you itemize, as long as your income is below the phase-out threshold. The saver's credit rewards retirement contributions for moderate-income filers with a credit worth up to $1,000. The premium tax credit helps marketplace health insurance buyers. And for 2025 returns, new temporary breaks like the car loan interest deduction (up to $10,000 for U.S.-assembled vehicles) and the overtime pay exclusion apply to millions of filers who may not even know they qualify.

What's the difference between a tax credit and a tax deduction?

A tax credit reduces your bill dollar for dollar. A deduction reduces your taxable income, which reduces your bill by a fraction of the deduction — determined by your marginal tax rate. A $1,000 credit saves $1,000. A $1,000 deduction saves $220 if you're in the 22% bracket.

What are the most commonly missed tax credits?

The earned income tax credit, worth up to $8,046 for 2025, goes unclaimed by millions of eligible filers each year. The saver's credit, the child and dependent care credit, and the American Opportunity Tax Credit for college costs are also frequently overlooked.

Is it possible to claim too many deductions and get audited?

Yes. The IRS compares deductions against income levels and flags returns where amounts look disproportionately large. Charitable deductions, home office expenses, and Schedule C losses are all monitored. Keep receipts and documentation for every deduction you claim.

Tax Software & Professional Help

For filers with straightforward W-2 income and standard deductions, good tax software is efficient, accurate, and often free. The IRS Free File program is available to taxpayers with adjusted gross income of $89,000 or less, and most major platforms offer guided filing for simple returns at no cost. NerdWallet's annual comparison of TurboTax, H&R Block, TaxAct, and TaxSlayer can help you identify which platform handles your specific situation — freelance income, investment sales, or rental properties — at the right price point.

The calculation changes when your return gets complicated. Anyone who owns a business, has significant investment activity, recently sold a home, received an inheritance, or experienced a major life event like marriage, divorce, or a new dependent generally benefits from a professional. A CPA or enrolled agent does more than data entry — they identify planning opportunities, catch overlooked deductions, and can represent you if the IRS comes calling. NerdWallet recommends verifying credentials through the IRS's preparer database, asking about year-round availability, and avoiding preparers who promise unusually large refunds or charge fees based on a percentage of your return.

When does it make sense to hire a tax professional instead of using software?

When your situation involves self-employment, investment sales, rental property, foreign accounts, an estate or trust, a business partnership, or significant life changes that affect eligibility for credits and deductions. The professional's fee often pays for itself in identified savings.

What credentials should a tax preparer have?

At minimum, a valid Preparer Tax Identification Number (PTIN) — required by law for anyone paid to prepare federal returns. Enrolled agents, CPAs, and tax attorneys hold higher credentials and can represent taxpayers before the IRS. Avoid anyone who won't sign the return they prepare.

Is year-round tax planning worth it or just for wealthy people?

Year-round planning is valuable for anyone whose income, investments, or life circumstances change significantly from year to year. Adjusting withholding mid-year, making strategic retirement contributions, timing investment sales, and catching deduction opportunities before December 31 are all moves available at any income level.

Tax Planning & Strategy

The biggest tax mistake of all is treating tax season as a once-a-year event. The decisions that determine your tax bill are made throughout the year — when you sell investments, contribute to retirement accounts, convert a traditional IRA to a Roth, or time the recognition of income and deductions. By the time you're sitting down to file in April, most of the leverage is gone.

Roth conversions are one of the most powerful planning tools available, particularly for people in low-income years — early retirees before Social Security kicks in, career changers, or anyone between jobs. Converting traditional IRA assets to a Roth means paying ordinary income tax now, in exchange for tax-free growth and withdrawals forever after, with no required minimum distributions during the owner's lifetime. Kiplinger has argued that the current tax environment, with the extended brackets from the One Big Beautiful Bill Act, represents a rare window for cost-effective Roth conversions through roughly 2028. But it's not a no-brainer — stacking a large conversion with capital gains harvesting in the same year can push your effective marginal rate far above what the bracket table suggests, as Social Security income becomes more taxable and Medicare surcharges phase in. Get the math right before you convert.

What is tax-loss harvesting and how does it reduce your bill?

Tax-loss harvesting is the practice of selling investments that have declined in value to generate capital losses that offset gains elsewhere in your portfolio. Up to $3,000 in excess losses can also offset ordinary income, and remaining losses carry forward to future years.

How does a Roth conversion affect taxes in the year you convert?

The converted amount is treated as ordinary income in the year of conversion, pushing your taxable income higher. This can affect your marginal bracket, trigger Medicare surcharges, increase the taxable portion of Social Security benefits, and reduce eligibility for certain income-tested deductions and credits.

What's the 0-percent capital gains strategy and who qualifies?

Filers whose taxable income falls below $48,350 (single) or $96,700 (married filing jointly) in 2025 pay 0% federal tax on long-term capital gains. Early retirees and those in low-income transition years can strategically realize gains — or do partial Roth conversions — to fill this bracket without triggering any tax on the investment profits.

Use these useful investment apps to help yourself out.

Keep Your Tax Research Organized With Miimu

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