It sounds as though this might be a question for a job application. If so, you should probably try and think of a situation related to your own experience, or more specifically, an experience that you relate specifically for the job you are applying for.
It doesn't have to actually have happened, or it can have it's basis in truth, but you can embellish certain aspects to suit the job that you want.
If you are applying for a job where you are likely to dealing with disgruntled customers, create a situation where you provided excellent service in a situation where someone was annoyed, but due to the measures you took, they left feeling much calmer and that their problem had been solved.
It is good to choose an example involving a challenge, and where your actions were over and above what was expected, and are relevant to the job you are applying for.