Everyone has had a situation in which a customer service interaction did not go as planned. Try to think of a situation that isn't too severe or the consequences weren't vast. If you tell the interviewer, you have never had a poor experience with a customer, then chances are you are not telling the truth.

Here is an interview answer example: "At my last role, I was an usher at a local movie theatre and a customer complained that the picture wasn't in the correct screen ratio for the previews. The movie started at the point when they came to complain, so I apologized and rushed upstairs to the projector room to correct the issue. Later on, they complained to corporate and said I should have done more. I agreed, I should have stayed after the movie ended and apologized again to the patron and if they were still unhappy offered them free tickets to their next visit."