This is a question that also tests honesty because if you have never encountered this issue, you shouldn't make something up. That doesn't mean that you won't have an answer. Have you ever personally been billed for something you didn't buy? Has a waiter ever charged you for something you didn't order? It's all about checking the work before the debt is paid.
If you have experience with this, relay that story and let them know exactly how you handled the situation.
"At my previous company, before paying any invoices, my manager and I double checked them before submitting them. When my manager was on leave over the holidays one year, I was allowed to approve with the instructions that if there was an error, to contact my manager's supervisor, the Director of Accounting. An invoice came in that was supposed t show a credit, but it hadn't been applied. In the notes for that vendor, there should have been a credit of four hundred dollars and change, but it wasn't applied and they were charging for the entire shipment. I immediately sent an email attaching the invoice and screenshot of the note in the vendor file and escalated it to the Director. We caught it in time and revised the vendor invoice to pay the appropriate amount, which helped our company save money during the holidays."
"When I started as the Accounting Manager for my last company, it was in shambles. The previous Accounting Manager had been fired for stealing from the company and there were almost no internal controls. As the company wanted to go public, it needed to have them. I found that invoices were submitted, approved, and processed by the same person. After reviewing their work, it was obvious they weren't actually reviewing the invoices before paying them. The company had been overpaying its vendors for six months and not getting the same amount of product. I quickly instituted internal controls to separate the duties involved with invoice payment and retrained the staff on how to handle these situations. After slight push back and a small turnover, we were back on track. I also contacted the vendors and obtained credits for the over-payments they'd received from us. This saved the company from paying invoices to those five vendors for the next two months."