Can a judge be called to testify as an expert witness on behalf of the other party?

There is, of course, an obvious danger undue weight might be accorded to the opinion of a judge. As one court held in the context of an insurance bad faith case where the trial judge was called to support the plaintiff's contention his defense was mishandled in a previous proceeding, "We think it prejudicial to one party for a judge to testify as an expert witness on behalf of the other party with respect to matters that took place before him .... [p] We conclude that the trial court erred in permitting the judge to testify as an opinion witness with respect to matters that had come before him in his judicial capacity." (Merritt v. Reserve Ins. Co. (1973) 34 Cal.App.3d 858, 882-883, 110 Cal.Rptr. 511.)