Tax filing mistakes rarely come from trying to cheat the system. They come from the genuine complexity of the tax code, incomplete information, and the pressure of a deadline. Here are the ones that appear most consistently — and what to do about each one.
Not Reporting All Income
The IRS receives copies of virtually every income document issued — W-2s, 1099-NECs, 1099-Ks from payment processors, 1099-INTs from banks, 1099-DIVs from investment accounts. Your return gets matched against these automatically.
If income appears on a document the IRS received and doesn't appear on your return, you'll receive a notice. If the discrepancy is large, it triggers further review.
Report everything. If you received a 1099 for $150 of interest income, it goes on the return. If you received freelance payments that weren't reported on a 1099 because they were under the reporting threshold — or because the client just didn't issue one — the income is still taxable.
Unreported income is one of the most common things that turn routine IRS inquiries into more significant problems.
Wrong Filing Status
Your filing status determines your tax bracket, your standard deduction, and your eligibility for various credits. Using the wrong one affects your entire return.
The status options are: single, married filing jointly, married filing separately, head of household, and qualifying surviving spouse. Each has specific requirements.
Head of household is frequently filed incorrectly. It requires that you're unmarried (or considered unmarried under IRS rules), that you paid more than half the cost of maintaining your home, and that a qualifying person lived in your home for more than half the year. People who simply live alone or who are separated — but not divorced — sometimes claim this status without meeting all the requirements.
Married couples should compare the tax outcome of filing jointly versus separately in certain situations — particularly when one spouse has significant medical expenses, student loan income-based repayments, or other income-contingent obligations where separate filing affects the calculation.
A second set of eyes catches the errors software doesn't flag. Have your return reviewed before it goes to the IRS.
Missing Deductions You're Entitled To
Missed deductions are common across multiple categories.
Self-employed people frequently miss: the home office deduction (which many avoid out of misplaced fear), the deduction for half of self-employment tax, self-employed health insurance premiums, and the qualified business income deduction.
Business owners miss: vehicle expenses when they don't maintain mileage logs, equipment purchases that qualify for Section 179 expensing, and meals and entertainment that were genuinely business-related.
Individual filers miss: student loan interest, educator expenses, contributions to HSAs, and deductible IRA contributions.
Each of these has specific requirements and documentation. The pattern is that people know broadly they exist but either don't know if they qualify or don't have the documentation ready.
Not Keeping Adequate Records
The IRS requires documentation for deductions. "I spent about $3,000 on business supplies" isn't a deductible claim — a receipt showing what was purchased and when is.
Good records are what make deductions defensible in an audit. Bank statements establish that a transaction occurred. Receipts establish what was purchased and establish the business purpose. A mileage log documents business driving.
The time to create documentation is at the time of the transaction, not when you're preparing your return. Trying to reconstruct a year's worth of expenses from memory and partial bank statements produces less accurate returns and less defensible deductions.
Missing the Retirement Contribution Opportunity
Many people contribute to retirement accounts during the year — but don't maximize contributions, or don't realize they can make prior-year contributions up to the filing deadline.
IRA contributions for a given tax year can be made up to April 15 of the following year. SEP-IRA contributions for sole proprietors can be made up to the filing deadline, including extensions. These are dollars that directly reduce your taxable income — they're among the most powerful deductions available.
If you haven't fully contributed to your IRA or SEP for last year, you have time to do so before filing your return. If you're not sure what you're eligible to contribute, that's worth a brief conversation with your accountant before you file.
Estimated Tax Underpayment
Self-employed people and business owners are required to make quarterly estimated tax payments. Missing these payments, or underpaying, generates an underpayment penalty.
The penalty isn't catastrophic — it's essentially interest at a modest rate — but it's avoidable. Making sufficient estimated payments throughout the year eliminates it entirely.
The safe harbor: pay 100 percent of your prior year tax liability (110 percent if your prior year AGI exceeded $150,000) in four equal installments on the quarterly due dates. This protects you from the penalty regardless of what you actually owe this year.
Mathematical and Transcription Errors
These are less common with tax software than they once were — software handles the math and transfers amounts between forms automatically. But transcription errors still happen, particularly when information is being entered manually from multiple documents.
Review your return before filing. Check that each 1099 and W-2 input matches the actual document. Verify that large numbers are entered correctly. Make sure your bank account number for any refund is accurate.
Before any return is filed, we check it systematically against every category above. What we're after is accuracy in both directions: everything you're entitled to, nothing you're not, and documentation that makes the return defensible.