The following legendary story of grandma going to court illustrates this concept in a funny and unexpected way:
Of course, in real life, attorneys are not usually afraid of the witness revealing embarrassing information about them, but are concerned the witness will say something unexpected about the evidence that effectively ruins the attorney's case. This can happen as a result of lack of proper preparation. Or, in criminal cases, it can happen because of little or no opportunity to talk to the witness before trial.
A victim or witness is not required to speak to or cooperate with the attorneys on either side of a criminal case. Unlike in a civil case, there are usually no depositions or interrogatories, or other opportunities to question a witness before trial. The witness may have been questioned to some degree during a grand jury proceeding or preliminary hearing. But often a criminal attorney has to rely on statements the victim or witness previously made to police or others.
That is why, in the field of criminal law, it is a myth to suggest that a lawyer should never ask a question to which she does not know the answer.
For an example of why an attorney should not ask a question without first knowing the answer, check out the Academy Award winning movie "Anatomy of a Murder" with George C. Scott and Jimmy Stewart: