If an order is placed on a site, using a product that uses inventory, the inventory is not deducted until the order has been approved. If an order item does not receive approval for some time, there is the chance that the inventory for that order item has been used whilte it was waiting for approval
This has happened several times to one of our customers. It is very frustrating as the order will not be processed. Also, If you have a large order with multiple items you have to try to figure out which item is the problem child. Inventory should be deducted when the order is placed and awaiting approval. If the order is not approved the inventory can be realocated.