Not reconciling bank or credit card accounts
Every bank and credit card account needs to be reconciled every month. Reconciling makes sure your accounting software is not missing any transactions, so your financial statements are complete and accurate.
If you miss expenses, you overpay in taxes. If you miss collections, your chances of getting an IRS letter go up. Either way, you end up paying more, in professional fees or in taxes.
The fix: reconcile every account, every month.