Unfortunately, as I got to know the Plaintiff, she revealed an incident of violence in the workplace which, if true, had the potential of completely changing the value of the case. The defendants were caught unawares and the negotiation was severely hampered, and ultimately unsuccessful given that this allegation had not been verified or even made a part of the pleadings by the time of the mediation hearing. There had been no discovery--formal or informal and the Defendants had no way of evaluating the veracity of a claim that was made for the first time during the first hour of a 1/2 day mediation hearing.
In this instance, the Plaintiff's attorney was also unaware of the new issue, so he could not have raised it sooner. Still, in negotiation, being prepared and preparing your client and your adversary, will always have a better chance of success than surprising the lawyers and parties with new facts and allegations at the time of the hearing.
That's my View from the Middle of the Road!