§ 40.330 Rule 605. Competency of judge as witness

The judge running a trial can't also be a witness in that same trial. This makes sense because a judge is supposed to be neutral and decide the case fairly. If the judge also testified, that would mix up two very different roles: deciding the facts and being a source of facts.

If a judge tries to testify in a case they're presiding over, the lawyers don't need to object to protect their client's rights on this issue. The problem is automatically preserved for appeal, even without anyone speaking up in the moment. This protects the fairness of the trial without putting lawyers in the awkward spot of having to challenge the judge directly during the proceedings.