I failed my checkride on a type rating, forgetting to start my time on a non-precision approach. As long as I can explain myself and prove that I learned from it, would Southwest have a problem with that?
Accept responsibility for the error (don't mitigate; "the DE was out to get me..."), give a BRIEF explanation for the gaff, explain how you've figured out not to do that again, tell them what you learned from the experience, how its made you a better pilot/person, how you've shared your experience and what you learned from it with others.
Turn the negative into a positive; "It made me a better pilot because.."
SWA likes real people, real people make mistakes. Real, good people learn from their mistakes and are willing to share with others.
Schedule an interview prep with Rob Beeks: (480) 575-9310. He covers these topics in depth.
I also failed a checkride, but it never came up at the interview. SWA is looking for reasons to hire you, most of the questions they ask are designed to give you an opportunity to show positives about yourself.
If it does come up, just follow InHot's advice. I would also recommend Rob Beeks for the interview prep, he is the man!