One of the ways to spice up a debate is to let each candidate ask a question of his opponent. If candidates agree to that, they have to be ready for anything.
At the recent debate of the candidates for Virginia's attorney general, Republican Ken Cuccinelli put one to Democrat Steve Shannon that's probably not that hard: name each division of the attorney general’s office and explain what each does. With a question like that, a candidate is probably hoping his opponent forgets one, or mixes two up, or offers an answer that suggests incomplete knowledge of the duties they seek.