Yes, a judge can declare a mistrial if there is legal error committed during the trial or if something happens that the judge believes substantially prejudices the case. For example, Idaho Rule of Criminal Procedure, Rule 29.1 provides: “A mistrial may be declared upon motion of the defendant, when there occurs during the trial an error or legal defect in the proceedings, or conduct inside or outside the courtroom, which is prejudicial to the defendant and deprives the defendant of a fair trial.”