A good response is for the candidate to discuss one example of when they have been disappointed in the past in their career. A person may have accepted a job position they regret, or wished to progress faster in their career, as a few examples. The candidate should not only discuss why this was a disappointment, but how they recovered from it and found new hope moving forward.
"My biggest professional disappointment is that I did not get a raise in my past job. However, I realized down the line that the job was not the right one for me, and I am glad to be in the position I am now, applying at this company instead."