You're building an end-of-month payment run from a long stack of incoming invoices. The IT equipment you ordered came in at the price you were quoted. The facilities bill matches the agreement, line for line.
All decisions were made when the purchase order was approved. But you still have to open each bill, find the right PO, check the numbers, and approve it by hand. Forty bills in, they're all sitting in the same queue waiting for the same manual check, so nothing makes the two that don't match their PO stand out. This is exactly where things slip, despite all the hours you put in week after week.