The billing system on an enterprise project that I've inherited is a nightmare. The client complained that an aged receivable was being reported as 31-60 days overdue when it should be 0-30 days overdue. I checked the system, and discovered that an account had multiple invoices. One was due in May and one was due in July. The system took a large payment, fully paid the invoice due in July, and partially paid the invoice due in May. That seems wrong.

I checked the code and confirmed my suspicion. The system is ordering open invoices by their creation date, not their due date. The invoice due in May was created after the first invoice, but had a much smaller window to pay. I'll bet anything the original coder assumed all invoices would have the same window to pay.