Van Wie's Dictum: Never introduce a new element (topic, issue, agenda item, etc.) in a negotiation unless absolutely necessary.
Example 1: The sales meeting is slowly but surely arriving at it's intended outcome and just as the discussion of next steps begins, one of our team members starts a completely new topic, such as a product or service not previously mentioned in the meeting.
Example 2: Both sides have discussed costs and benefits of the product (or service) and there seems to be great understanding and agreement on how the product service solves the key problem identified by the prospect. Just as the deal is to be closed, a member of our team volunteers, without having been asked by the prospect, 6 other uses of our product.
You can easily imagine what happens next.