Running a small business means juggling sales, payroll, expenses, growth plans, and somewhere in the mix—taxes. The IRS has increased its focus on small business compliance in 2025, especially around payroll taxes, digital payments, worker classification, and estimated tax payments. Most tax errors aren’t intentional. They happen because business owners are moving fast, wearing too many hats, or relying on outdated assumptions about what’s required.
The problem is that a small mistake—missing a quarterly tax payment, depositing payroll taxes late, or mixing personal and business expenses—can quickly lead to penalties, interest, or an unwanted IRS notice. This guide covers the most common tax mistakes small businesses make today, what’s changed for the 2025 tax year, and how to stay ahead of issues before they become expensive.
Table of Contents
- Missing estimated tax payments
- Mixing personal and business expenses
- Poor recordkeeping
- Filing late or misunderstanding extensions
- Misclassifying workers
- Payroll tax mistakes
- Incorrect or missing 1099s
- Underreporting income
- Missing deductions and credits
- Ignoring state and local tax rules
- How Harness can help you
1) Missing estimated tax payments
Many small business owners get caught off guard by estimated taxes. If you expect to owe $1,000 or more in taxes for the year and don’t pay throughout the year, the IRS charges penalties and interest. This applies to sole proprietors, LLC owners, freelancers, and anyone who doesn’t have taxes withheld from a paycheck.
For 2025, estimated taxes are due four times a year—April, June, September, and January. The IRS also increased its interest rate on underpayments again in 2025, which makes missing or paying late even more expensive.
There are safe harbor rules that can help you avoid penalties. Generally, you won’t be penalized if you pay at least:
- 90% of the tax you owe for the current year, or
- 100% of your previous year’s tax liability (110% if your income was over $150,000)
What makes this a common mistake is that income for small businesses isn’t always predictable. Revenue fluctuates, quarterly payments get postponed, and owners sometimes assume they can “catch up” at filing time. But the IRS calculates penalties from the moment the payment was due—not when the return is filed.
Setting aside tax money monthly, using accounting software to project profits, or working with a tax advisor to calculate quarterly amounts can make this process more manageable.
2) Mixing personal and business expenses
One of the most common and costly mistakes small business owners make is combining personal and business spending in the same bank account or credit card. It may seem harmless (especially for sole proprietors or new LLCs), but it creates real problems at tax time.
When personal and business expenses are mixed together, it becomes difficult to prove which costs are legitimate deductions. That increases the chances of errors, missed write-offs, or worse—an IRS auditor questioning whether your business is being run as a real business at all.
It also makes accurate bookkeeping harder. Software like QuickBooks, Xero, or even a simple spreadsheet works best when business transactions are separate and clean. When an owner has to manually separate groceries from software subscriptions or personal travel from business trips, mistakes get made, and deductions are lost.
The fix is straightforward: open a dedicated business bank account and business credit card, even if you’re a single-member LLC or sole proprietor. Pay yourself from the business instead of paying bills directly from the business account. This one change makes it easier to track spending, calculate taxes, and support deductions if the IRS ever asks.
3) Poor recordkeeping
Disorganized or incomplete records are one of the fastest ways to lose deductions or draw IRS attention. Missing receipts, mileage logs, invoices, payroll reports, or proof of business purchases make it difficult to back up deductions if questioned. The IRS requires businesses to keep financial records for at least three years, but certain records—like payroll taxes, real estate, or asset purchases—should be kept longer. Cloud bookkeeping tools, digital receipt scanners, and automated bank feeds make recordkeeping easier and reduce errors. The businesses that run into problems are usually the ones trying to piece everything together at the end of the year instead of maintaining accurate books throughout.
4) Filing late or misunderstanding extensions
Business tax deadlines vary depending on structure—S corporations and partnerships typically file by March 15, while sole proprietors and C corporations file by April 15. Extensions give you more time to file the return, but not more time to pay taxes owed. Penalties apply separately: a failure-to-file penalty (5% of unpaid tax per month) and a failure-to-pay penalty (0.5% per month). Filing on time, even if you can’t pay everything, helps reduce penalties. Many small businesses assume an extension covers everything and are surprised when interest and penalties start accumulating.
5) Misclassifying workers
The IRS and Department of Labor tightened enforcement in 2024–2025 around worker classification. Treating an employee as an independent contractor to avoid payroll taxes, benefits, or workers’ compensation can lead to back taxes, unpaid Social Security and Medicare contributions, fines, and legal exposure.
The IRS uses a “common law” test—looking at behavioral control, financial control, and the nature of the relationship. If you’re unsure, Form SS-8 can be filed for a determination. Misclassification is especially common in startups, construction, consulting, and creative industries.
6) Payroll tax mistakes
If you have employees, you’re responsible for withholding federal income tax, Social Security, and Medicare taxes—and paying the employer portion of Social Security and Medicare. These must be deposited using the Electronic Federal Tax Payment System (EFTPS). Missing a deposit is costly: penalties range from 2% to 15% depending on how late the payment is. In more serious cases, the IRS can apply the Trust Fund Recovery Penalty, holding the business owner personally liable. Using payroll software or working with a payroll service provider helps prevent late deposits and incorrect filings.
7) Incorrect or missing 1099s
Businesses must issue Form 1099-NEC to contractors paid $600 or more during the year. In 2025, platforms like PayPal, Cash App, and Stripe are still required to issue 1099-Ks only if payments exceed $5,000, after the IRS delayed the $600 threshold again. Starting in the 2025 tax year, digital asset exchanges will issue Form 1099-DA for cryptocurrency transactions. Missing or incorrect forms can trigger penalties per form and can flag your return for review if the income reported by contractors doesn’t match IRS records.
8) Underreporting income
Underreporting income (whether cash, digital payments, or third-party transactions) is one of the most common audit triggers. The IRS now receives income data from payment processors, brokerage firms, gig platforms, and crypto exchanges. If your return doesn’t match what’s reported to the IRS, a notice is almost guaranteed. Even small cash-based businesses are facing more scrutiny, as the IRS has expanded data matching technology. Accurate accounting and reconciliation throughout the year help prevent unintentional underreporting.
9) Missing deductions and credits
Small businesses often leave money on the table by skipping legitimate deductions and credits. Section 179 expensing and bonus depreciation allow businesses to immediately deduct qualifying equipment. Home office deductions are allowed if the space is used regularly and exclusively for business. Retirement contributions—such as SEP IRAs or Solo 401(k)s—can reduce taxable income while building savings. The R&D credit, energy-efficient commercial building deductions, and qualified business income (QBI) deduction still apply to many businesses in 2025. These are often missed simply because records aren’t kept or business owners aren’t aware they qualify.
10) Ignoring state and local tax rules
State and local tax compliance has become more complex—especially for businesses with remote employees, online sales, or operations in multiple states. Nexus rules determine where a business owes tax, and having just one remote employee in another state can create a filing requirement.
Many states continue to offer pass-through entity tax (PTET) elections as a workaround for the federal $10,000 SALT deduction cap. Localities may also impose business income taxes, gross receipts taxes, or city licensing fees. Ignoring these requirements can lead to back taxes and penalties at both the state and local levels.
How Harness can help you
Most small business tax mistakes don’t come from neglect, they come from moving fast, focusing on growth, and not having the right systems or people in place. But taxes don’t have to be stressful or reactive. Getting organized early, planning before year-end, and working with an advisor who understands your business can help you avoid penalties and make better financial decisions all year, not just at filing time.
If you want to get ahead of tax season, make smart planning moves, or feel confident your return is done right, you can work with a vetted advisor through Harness.
Get started with Harness today.
Disclaimer:
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