Everyone has had a failure at one point in their career. Do not hesitate to describe a failure. The key to a great answer, is how you learned from that failure and were able to improve yourself because of it. Try to keep the failure to one instance and stay away from financial mistakes you made at your previous employer. Example, do not tell the interviewer you forgot to order new inventory and which costs your company fifteen thousand in lost sales. Instead, try to keep it career related. Example, I was offered a position at XYZ that would have gained me management experience in my career, but I turned down the offer in fear I was not yet ready for the position. In hindsight, I believe I was more than capable to run XYZ's department, and regret not accepting.