Schedule C reports your business profit or loss to the IRS. The numbers on that form come directly from your books, so when the bookkeeping has problems, the tax return has problems. These are the mistakes that cause the most issues.

Mixing personal and business expenses is the most common problem. Personal purchases running through your business account inflate your deductions and create a mess at tax time. Your tax preparer either has to review every transaction to pull out personal items or file with inflated expenses that could trigger an audit. Use separate accounts for business and personal spending.

Missing or incomplete mileage records create trouble if you claim vehicle deductions. The IRS expects a contemporaneous log showing the date, destination, business purpose, and miles driven. Reconstructing this at year end from memory doesn’t hold up well, and claiming mileage without records invites problems during an audit. If you use your vehicle for business, track miles throughout the year with an app or simple log.

Home office deductions require documentation that many business owners don’t maintain. You need to know the square footage of your dedicated workspace and your total home square footage. If you’re using the actual expense method instead of the simplified method, you also need records of housing costs like mortgage interest, property taxes, utilities, and insurance. Taking the deduction without this support is risky.

Incorrect cost of goods sold distorts your entire return if you sell products. COGS requires calculating beginning inventory, purchases during the year, and ending inventory correctly. Businesses that don’t track inventory properly often dump all purchases into COGS without accounting for what’s still on hand at year end. That’s both inaccurate and could understate your taxable income.

Uncategorized cash payments cause two problems. Cash you pay out that never gets recorded means you’re missing legitimate deductions. Cash you receive that never hits the books means you’re failing to report income. The IRS has methods to detect unreported cash income, and the penalties for underreporting are significant. Deposit all cash receipts and record every cash payment with proper documentation.

Missing 1099 income triggers IRS notices because they receive copies of every 1099 issued to you. If your books don’t reflect that income, you’ll hear about it. This happens when 1099s arrive after you’ve already closed the year, when you receive payments through multiple platforms, or when income gets miscategorized. Reconcile all 1099s against your records before your business tax return gets filed.

Most of these mistakes happen because bookkeeping gets neglected during the year. By tax season, you’re reconstructing records from memory and bank statements, which is how errors slip through. If your books already have these issues, bookkeeping cleanup and catch-up services can sort them out before filing. And once clean, maintaining the books monthly keeps next year’s Schedule C straightforward.