Which of the following is NOT a reason for a witness to be impeached?

In the context of witness impeachment, the goal is to challenge the credibility of a witness to affect their reliability in the eyes of the court. Actions such as prior bad acts, contradictions by other witnesses, and evidence of bias or prejudice are all legitimate grounds for impeachment as they directly relate to a witness's character or reliability.

Expertise in the field does not serve as a reason for impeachment. In fact, a witness having expertise can actually enhance their credibility, as it suggests they possess specialized knowledge that may lend weight to their testimony. Therefore, having expertise does not detract from a witness's credibility, unlike the factors mentioned in the other choices, which can discredit a witness and impact the overall testimony rendered in a case.