Rule 605 Competency of Judge as Witness

The judge in charge of a trial cannot also be a witness in that same trial. If the judge has information relevant to the case, they need to step aside and let another judge handle the trial instead.

This rule exists to keep things fair. A judge is supposed to be a neutral referee who listens to evidence and makes rulings on the law. If the judge also testified as a witness, they would be mixing two different jobs that need to stay separate. The judge would end up judging their own testimony, deciding how believable it is, and ruling on objections to their own words. That puts too much power in one person's hands and makes it hard for either side to get a fair trial.

Because of this, nobody even needs to formally object if a judge tries to testify. The problem is automatic and does not require anyone to raise their hand and complain about it during the trial.