This question often comes up to gauge your stress levels through your reaction. It is likely, though, that there is no reason for concern when this question comes up. In most states, it is illegal for a prospective employer to ask your current or past employer anything other than, would you hire them again? Additionally, a recruiter will probably go through the motions of calling previous employers, references, and performing background checks once they've decided who they are interviewing for their second or third round. Keep your composure, exuding confidence and a friendly demeanor as you take advantage of this opportunity to advertise your merits. Think about the positive things your past employers have said about you. Make your answer unique and memorable.