Many interviewees freeze at this question, simply because they cannot think of an example, rather than the fact that they have never dealt with one. So have an answer prepared and make sure it is one where you resolved the issue, not one where you had to refer the customer to a higher authority (it’s amazing how many people do this). What the interviewer is looking for are the skills you possess in handling difficult customers, not the intricate detail of the particular issue the customer had.