Graduate recruiters use some questions as much to see how you cope when you're put on the spot as to elicit a truthful answer. This question is a test of your ability to think on your feet and come up with a diplomatic response. Whether you sidestep the question by saying you've always got on well with your employers, or describe a tricky situation you've experienced that highlights your potential, you need to avoid attacking your previous employers. Also, take care not to incriminate yourself.