Whether a circuit court judge's dual role as a witness and sentencer in the same case creates a conflict and reversible error when the judge imposes an illegal sentence that violates due process

QUESTION(S) PRESENTED ; When a circuit court judge is called upon, to be a witness against a defendant in a case, and then later on, sentence the defendant in the same case right afterwards. Does this create a conflict with the defendant and the judgment entered? After sentencing, does trial court commit reversible error when it sentences a defendant to an illegal sentence and is clearly shown. Does the court create a manifest injustice and a violation of due process when it refuses to correct an illegal sentence? 2