I'll be the first to admit that my Accounting knowledge isn't where it should be (given my position at my company), but I saw something in NS that raised my eyebrows.

Our default SO form is a "cash sale form". A fulfilled order had inadvertently been billed as a "cash sale" and with NO payment method or "null" as the payment method.  The customer called a month later and said "I got my stuff, but you never billed us".  I looked and saw that it was fulfilled and marked billed and basically closed.  Our support person had entered an order for a customer with terms, but inadvertently left it on "cash sale" and didn't pick a payment method.