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Top 10 Tax Mistakes That Cost Americans Thousands

Tax season should be a chance to reclaim money the government held on your behalf throughout the year. But for millions of Americans, it becomes a source of stress, penalties, and missed opportunities. Many of the most expensive tax errors are completely avoidable.

Here are the ten most common tax mistakes that cost people real money — and how to avoid each one.

Mistake 1: Missing Deductions You Qualify For

The IRS does not remind you what you are entitled to deduct. If you do not claim it, you lose it.

Common missed deductions include:

State and local income or sales taxes (up to $10,000)
Mortgage points paid when refinancing (deductible over the loan term)
Medical expenses above 7.5% of adjusted gross income
Charitable mileage (14 cents per mile for charity driving)
Investment losses that can offset gains or reduce income
Job search expenses in certain circumstances
Energy-efficient home improvements (eligible for tax credits)

Solution: Use tax software that asks detailed questions, or work with a tax professional who probes beyond the basics.

Mistake 2: Filing the Wrong Status

Your filing status determines your standard deduction, bracket thresholds, and eligibility for many credits. Filing the wrong status is one of the costliest mistakes you can make.

The options are: Single, Married Filing Jointly, Married Filing Separately, Head of Household, and Qualifying Surviving Spouse.

Head of Household is commonly misused. To qualify, you must be unmarried, have paid more than half the cost of keeping up a home, and have a qualifying dependent living with you for more than half the year. This status gives a larger standard deduction and lower bracket rates than Single.

If you are divorced or legally separated and have a child who lives with you, you may qualify for Head of Household — but many people incorrectly file as Single and leave money on the table.

Mistake 3: Overlooking the Earned Income Tax Credit

The Earned Income Tax Credit (EITC) is one of the most valuable credits for low-to-moderate income workers, yet the IRS estimates that roughly 20% of eligible taxpayers never claim it.

For 2026, the maximum EITC is:

$632 with no children
$4,213 with one qualifying child
$6,960 with two qualifying children
$7,830 with three or more qualifying children

Eligibility depends on income, filing status, and number of children. Many people assume they do not qualify without actually checking. Use the EITC Assistant on IRS.gov to see if you are eligible.

Mistake 4: Not Reporting All Income

This is a mistake that can cost you much more than just a missed deduction — it can trigger penalties, interest, and even criminal charges.

Income you must report includes:

Freelance and gig economy income (even if you did not get a 1099)
Rental income
Side business income
Gambling winnings
Barter income (the fair market value of goods or services received)
Forgiven debt (which can be taxable as cancellation of debt income)
Social Security benefits (if your combined income exceeds certain thresholds)

The IRS receives copies of every 1099 and W-2 issued in your name. They will catch discrepancies. Report everything, even if you do not receive a form.

Mistake 5: Ignoring Retirement Contribution Opportunities

Many employees contribute to their 401(k) but never increase their contribution as their income grows. They also miss:

IRA contributions: If you are eligible, a traditional or Roth IRA contribution made by the tax filing deadline (April 15) can reduce taxes for the prior year.
HSA contributions: Same deadline applies — contribute up to the limit before filing and claim the deduction.
Catch-up contributions: If you are 50 or older, you can contribute extra to your 401(k), IRA, and HSA.

Retirement contributions are one of the most powerful tools for reducing taxable income. Not maxing them out is leaving money in the government’s hands.

Mistake 6: Not Keeping Records

You can claim all the deductions you want, but without documentation, they will not survive an audit. The IRS requires substantiation.

Keep records of:

All business receipts
Mileage logs
Charitable donation receipts (required for all cash donations and for any non-cash donation over $250)
Investment records (cost basis, dates acquired and sold)
Home improvement receipts (relevant if you sell your home)

The IRS generally has three years to audit a return, six years if they suspect substantial underreporting, and no limit if they suspect fraud. A good filing system protects you.

Mistake 7: Filing Late or Not at All

Missing the filing deadline costs money in multiple ways:

Failure-to-file penalty: 5% of your unpaid taxes per month, up to 25%.

Failure-to-pay penalty: 0.5% of unpaid taxes per month, up to 25%.

Interest: Charged on top of unpaid taxes from the original due date.

If you cannot file on time, request an automatic six-month extension (Form 4868). This extends the filing deadline but not the payment deadline. If you owe taxes, estimate and pay as much as you can by April 15 to minimize penalties.

Even if you cannot pay, file the return. The failure-to-file penalty is much higher than the failure-to-pay penalty.

Mistake 8: Making Math and Transcription Errors

Manual errors — wrong Social Security numbers, transposed digits, and arithmetic mistakes — are among the most common reasons the IRS rejects or adjusts returns.

These errors can:

Delay your refund
Trigger IRS notices
Result in incorrect tax amounts

Solution: Use tax software, which handles all calculations automatically and performs error checks. If filing on paper (which is rare now), triple-check every number you enter.

Mistake 9: Not Considering the Alternative Minimum Tax

The Alternative Minimum Tax (AMT) is a parallel tax system designed to ensure high-income earners pay a minimum amount of tax. It was originally designed to prevent wealthy people from using excessive deductions to zero out their tax bills.

Today, however, the AMT can catch some middle-income earners, particularly those who:

Exercise incentive stock options (ISOs)
Have large deductions for state and local taxes
Have significant miscellaneous deductions
Have a large number of personal exemptions

If you fall into these categories, you may owe additional AMT on top of your regular income tax. Planning around the AMT requires professional guidance.

Mistake 10: Not Getting Professional Help When Needed

Tax software is excellent for straightforward returns. But it cannot replace expertise when your situation is complex.

Consider working with a CPA or enrolled agent if you:

Are self-employed with significant income or expenses
Own rental property
Have significant investment activity
Recently went through a major life change (marriage, divorce, inheritance, home sale)
Received a large bonus or equity compensation
Are starting or closing a business

The cost of professional tax preparation is itself tax-deductible as a business expense, and a skilled tax professional can often find savings that far exceed their fee.

The Bottom Line

Tax mistakes are expensive. The ones on this list cost Americans billions of dollars every year — in missed refunds, unnecessary penalties, and overpaid taxes.

The good news is that every single one is avoidable. Start with good recordkeeping, understand the deductions and credits you qualify for, file on time, and seek professional help when your situation warrants it. Your tax bill is within your control far more than most people realize.