This is likely caused by having deleted a product group from a quote before the products within it. There is a bug within version 7 of CRM that will leave the quantities on there. Should you be on version 7, your best business practice should you need to delete a product group from a quote is to delete the items from within it first. To rectify the erroring quote, please see the following Get support in Access CRM and reference the title of this article.
