It's all about wording. No one is perfect, and there will always be flaws in the past that will make you seem unsuitable for a position. What you need to do is find a way to put this into a positive light - speak to how you learned from it, how it has changed your viewpoint, changed your way of thinking/acting, or whatever.
As an example, when I was doing my undergrad I did poorly in one of my math courses that is quite important for grad school. So of course, in my interviews for grad school, I was asked about this everytime. The first few interviews I didn't have a well crafted response and it showed, and I was turned down or waitlisted by the schools. Eventually, I realized I needed to figure out a way of wording my answer in a way that put me in a positive light, while still being honest.
Once I put in some serious time into crafting a response, bounced it off friends and family to get their thoughts, the interviews became a breeze. I felt more confident, I was more comfortable in the interview, and I wasn't afraid of being asked tough questions. People have said it before and I'll say it again, preparation is everything. Once I had done this, I was getting offers instead of being waitlisted everytime.
Also, one question that I find is a really good one to ask during the time you're given to ask questions back is: "At this point, what is the biggest thing that would stop you from hiring me?" It takes balls to ask a question like that, but your potential employer will respect you for having the guts to do it. It also gives you a chance to clear up any misconceptions that the interviewer might have about you.
Good luck, and hopefully the next one goes better!