Everyone makes mistakes, and no one is perfect. The interviewer knows that too and is asking this question to see how you handle situations when you make mistakes. Give an example that demonstrates how you took responsibility for your error and fixed it. Also, what you learned from the experience as a result. Sometimes learning from our mistakes allows us to gain some valuable tools.
"When I worked as a financial services rep, I accidentally double booked two clients for an appointment in one of the time slots. I realized the mistake after I was double checking my calendar and called the clients right away and apologized for my mistake. As a result of this experience, I always made sure to check the time slots on my calendar twice."
"When starting my current position, I had some struggles with learning Salesforce and let the frustrations get the best of me. I felt like a failure and was ready to give up. Once my boss pulled me aside and reminded me that everyone struggles with brand new technology, I was able to relax. I ended up taking an online course on the program so that I could fast-track my learning and avoid further frustration. What I learned was that only I am responsible for my learning and there are always ways to overcome challenges, if you look for the answers and are prepared to work for them."