Elon Musk has repeatedly been quoted as saying, "A failure is an option here. If things are not failing, you are not innovating."

This interview question focuses less on the fact that you failed and more on your emotional intelligence and whether you learned from that failure. Taking your failure in stride and finding the lesson to be learned from your failures ensures you are less likely to repeat your mistakes. There's an expression that says no one can hold over you what you already own. There is nothing to fear in admitting to the mistakes you've made. Consider this when formulating your answer.

"I failed designing a custom motor shaft. The shaft diameter was supposed to be 1.375" and I reversed the numbers to 1.357". I had to quickly design a sleeve that fit over the existing shaft to avoid a 100,000 loss and 3 month delay. This taught me to be more diligent when I release drawings."

"I once attempted to tackle a project far too quickly. The project was designing and building a small desktop lathe for winding coils. I tried to tackle the entire thing in a period of two days and as a result released a lathe (controlled by an FPGA) with several bugs in it. This would cause the lathe to react strangely at times and to even turn on its own sometimes."