I think there must be a circulating online class called "How to Interview an Ad Agency." The reason I say this is that during the past three RFPs, I've had potential clients challenge me with the same question: "Can you tell me about a campaign you did that failed?"
I have to think. Hard. Long silence for effect.
"No, I can't."
They've all worked; some better than others, but all advertising ended up bringing my clients more business than they had before they advertised. This isn't to be egotistical, because, quite frankly, any advertising, even dull advertising, has more of an impact than none at all. There is plenty of experience over the past couple of hundred years to support this. Business schools have even done studies to verify what we all know instinctively; advertising--any advertising--always works. Sometimes there are unintended consequences; sometimes the response isn't a tidal wave of enthusiasm, but word gets out more than if you had done nothing.
That's not to say that asking an interviewee to be reflective about his or her failings isn't a good technique. For one thing, you can tell if they're blowing smoke or being defensive. All good interviewees know to keep a stock story in their pocket for just such a question. And then they make sure that the story of failure is relative--mine is that my only regret was that I didn't stop the Vietnam War earlier than I did.
But I guess, for advertising, failure would be the difference between a 2% sales increase vs a 20%. Or that the total increase in sales amounted to less than the cost of the advertising. But in my experience, I've never encountered that. I'm that good. Or that cheap.
And that is egotistical.