Where errors come from
Poor-quality images, handwritten receipts, foreign-currency documents, one payment covering several invoices, and new types of expense the system has not seen. Errors also come from rules that were set up wrongly at the start and then applied consistently, which is why the first weeks need closer checking.
Controls that work
Review every new supplier and category the first time. Have the system flag low-confidence items rather than post them. Reconcile to the bank every week or month. Compare monthly figures with the same month last year to spot outliers. Keep an audit trail of who or what made each change, so any error can be traced and the rule fixed.