Tell the hiring manager about the most important lesson you have learned in your career so far. Try to tie your answer in to something that they can relate to.
"The biggest lesson that I learned as a QC Inspector was to be extra choosy when approving raw materials before production. One incorrect choice can alter the quality of the end product greatly. I made that error once, in the beginning of my career, and will never do it again. It was a tough but important lesson to learn."