A defense attorney may impeach a police officer who testifies regarding pending disciplinary action or pending criminal charges.  The defense attorney may cross-examine the witness with regard to the fact that the witness is facing or could potentially face criminal charges or disciplinary action, and use that to argue that witness may be testifying favorably on behalf of the Commonwealth to garner favor with his police department or the prosecutors involved in his case.  The defense attorney does not have to prove that the witness is receiving any preferential treatment, just that someone could infer the witness’s motive to hope for preferential treatment for his favorable testimony.