Everyone makes mistakes; character is defined by how you deal with them. This question will allow you to first gauge the candidate’s honesty. If they say that they’ve never made a mistake, you can rest assured that they’re not being truthful and their resume can go into the circular file. However, when they tell you about the mistake they’ve made, note if they take responsibility for it (that will reveal their level of maturity) and, of course, how they resolved it.