Most tax preparation mistakes are not exotic. They are basic execution failures that compound over time and become visible only when something goes wrong: an IRS notice, a bank refinance, an audit, a sale.

The list below is what we see most often when reviewing prior-year returns for new clients. None of these are obscure. All of them are avoidable with proper preparation.

Mixing personal and business expenses without documentation

This is the most common mistake we see, and the most costly when it surfaces.

The IRS does not accept a claim that an expense was “obviously business-related” without supporting records. Bank statements showing the charge are not enough on their own. The substantiation rules under Section 274 require documentation of the business purpose, the date, the amount, and (for meals and travel) the parties present and the business relationship.

If you cannot prove it, you cannot deduct it. The right time to set up clean separation between personal and business accounts is at the start of the activity. Cleaning it up later requires reconstruction work that often cannot fully recover the support the IRS expects.

Filing one Schedule C for multiple unrelated activities

If you run a consulting practice and a side e-commerce business through the same Schedule C, you have made future planning meaningfully harder.

You cannot see profitability by activity. You cannot evaluate whether one activity supports an S-Corp election while the other does not. If the IRS questions any portion of the return, the entire combined activity comes under review.

Unrelated activities should be reported on separate Schedule Cs at minimum, and ideally held in separate entities with separate books. The cost of doing this correctly is small. The cost of fixing it after several years of co-mingled records is not.

Ignoring estimated tax requirements

Self-employed taxpayers, S-Corp owners, and anyone with material non-W-2 income are required to make quarterly estimated tax payments. The safe harbor rules require paying either 90% of the current-year liability or 100% of the prior-year liability (110% if prior-year AGI exceeded $150,000) to avoid underpayment penalties.

Most underpayment situations are not about cash flow. They are about not having a projection. Without quarterly tax planning, the size of the April 15 balance is a surprise. Underpayment penalties run at the federal short-term rate plus 3 percent and compound daily.

The fix is straightforward: a quarterly projection that adjusts for actual income and a payment cadence that aligns with it.

Treating S-Corp reasonable compensation as a number to minimize

Reasonable compensation for an S-Corp owner-employee is what the IRS considers fair pay for the work performed, based on industry standards, qualifications, and time invested. It is not a number you choose to minimize self-employment tax.

An unreasonably low salary paired with large distributions is a documented audit trigger. When challenged, the IRS reclassifies distributions as wages and assesses payroll taxes, penalties, and interest. The cleanup is rarely affordable.

The “$50,000 rule of thumb” you may have heard does not stand up under examination. Reasonable comp depends on the work, the industry, and the income. We have a longer post on this: Why Your S-Corp Might Be Costing You More Than You Think.

Missing depreciation on rental property

Many DMV property owners file Schedule E year after year without claiming depreciation, or claiming it incorrectly. The IRS considers depreciation on rental real estate to be an allowable deduction whether you take it or not. When the property is sold, the IRS assumes you took the depreciation and assesses recapture tax on it regardless.

That is the worst possible outcome: you did not get the deduction during the holding period, but you owe tax on it at sale.

The fix is Form 3115, an Application for Change in Accounting Method. It allows missed depreciation to be captured in a single year’s return through a Section 481(a) adjustment. We see this come up regularly with multi-property owners who have held properties for five years or more without a depreciation review.

Filing in only one state when client work spans multiple jurisdictions

This is a particular DMV problem. A consultant who lives in Maryland, has clients in DC, and works on-site in Northern Virginia can have filing obligations in all three jurisdictions. The sourcing rules differ, the credit mechanics differ, and what each state considers nexus differs.

Filing only in the resident state may be defensible in some scenarios and a problem in others. The right time to make that determination is during planning, not at audit. State notices typically arrive several years after the underpayment, by which point penalties and interest have accumulated.

Not reconciling 1099s and W-2s before filing

The IRS receives copies of every 1099 and W-2 issued in your name. When the totals on your return do not match the third-party reporting, the IRS automated matching system flags the return and issues a CP2000 notice proposing additional tax.

The CP2000 is not a final assessment — you have the right to respond, document, and dispute. But the easier and cheaper path is to reconcile before filing: pull a wage and income transcript from IRS.gov, match each 1099 and W-2 to your records, and file a return that reflects what the IRS already knows about.

Filing without a strategy conversation

The single largest preparation mistake is treating tax filing as a paperwork exercise rather than the output of a planned year.

By the time the return is being prepared, most of the decisions that affect the outcome have already been made. The S-Corp election was either filed or it was not. The retirement contributions were either funded or they were not. The depreciation method was either elected correctly or it was not. The estimated payments were either accurate or they were not.

Preparation is the documentation step. The leverage sits in planning, which happens earlier in the year. If your tax relationship is filing-only, you are paying for documentation without strategy.

What we do differently

Our preparation engagements are tied to planning. We review the prior year for missed positions before filing the current year. We coordinate estimated payments with quarterly projections. We confirm depreciation, basis, and entity elections are tracking correctly across years. We catch most of the mistakes above before they become problems on the return.If any of the patterns above look familiar from your prior return, book a complimentary 15-minute strategy call to walk through your situation.