To address this anomaly, the Drafters added Section 5(b), a preclusion provision to cover situations in which the parties do not expressly waive the privilege but engage in conduct inconsistent with the assertions of the privilege, and that cause prejudice. As under existing interpretations for other communications privileges, waiver through preclusion would not typically constitute a waiver with respect to all mediation communications, only those related in subject matter. See generally Unif. R. Evid. R. 510 and 511 (1986).

Critically, the preclusion provision applies only if the disclosure prejudices another in a proceeding. It is not intended to encompass the casual recounting of the mediation session to a neighbor that is not admissible in court, but would include disclosure that would, absent the exception, allow one party to take unfair advantage of the privilege. For example, if one party's attorney states in court that the other party admitted destroying evidence during mediation, that party should not be able to block the use of testimony to refute that statement later in that proceeding. Such advantage-taking or opportunism would be inconsistent with the policy rationales that support continued recognition of the privilege, while the casual conversation would not. Thus, if Andy and Betty were the parties in a mediation, and Andy affirmatively stated in court that Betty admitted destroying evidence during the mediation, Andy is precluded from asserting that A did not waive the privilege. If Betty decides to waive as well, evidence of Andy's and Betty's statements during mediation may be admitted.

Analogous doctrines have developed regarding constitutional privileges, Harris v. New York, 401 U.S. 222, 224 (1971) (shield provided by Miranda cannot be perverted into a license to use perjury by way of a defense, free from the risk of confrontation with prior inconsistent utterances), and the rule of completeness in Rule 106 of the Uniform Rules of Evidence, which states that if one party introduces part of a record, an adverse party may introduce other parts when to do otherwise would be unfair.

Finally, it is worth noting that in arbitration, which is sometimes conducted without an ongoing record, it will be important for waiving parties to ask the arbitrator to note the waiver. Any individual who wants notice that another has received a subpoena for mediation communications or has waived the privilege can provide for notification as a clause in the agreement to mediate or the mediated agreement.