Mistakes are a part of the human nature, which is why it's so important to be diligent. I double check all of my work - doing it once on the computer, and a second time by hand. If they match, I'm sure that I have the right answer.

Accounting is all about money, and the last thing a business wants is someone making a careless (or even just not-careful-enough) mistake with that money. The interviewer wants to know that you have a system in place to make mistakes very rare.