So how many times has this happened to you? 

You are involved in a  negotiation. Everything seems to be going pretty well – your negotiation  styles and negotiating techniques seem to be working. A deal is  starting to form up and both sides are starting to see what they may be  able to walk away from this deal with when all of sudden something changes.  A spoiler enters into the negotiation. Once this happens, something  almost magical happens (in a bad way) and the deal that had looked so  close jumps out the window and runs away never to be seen again. 

Clearly  a deal spoiler is not something that any of us want to invite to a  negotiation; however, what can we do to prevent it from showing up  unannounced?