Everyone handles the stress and disappointment of setbacks differently. By asking this question, your interviewer aims to gauge your level of emotional intelligence. In other words, they want to know whether you can think quickly on your feet and adjust and adapt in such situations or whether you allow your emotions to govern your responses and actions. For example, those who feel disappointment, shame, or overwhelmed when things don't go according to plan are more likely to experience task paralysis or externalize their stress in such situations. So, when answering this question, you want to walk your interviewer through your approach, perceiving such setbacks professionally while taking decisive action towards making the best of such situations and continuing to move forward.