In short, the key with "failure" questions is to show what you learned and (perhaps even more importantly) then go on to describe a time when you put that lesson to use in another situation. Admissions officers want to hear about thing that you really did and really experienced, not hypothetical situations.
So, it's not enough to say "I learned that I should always get buy-in from the whole team, and from now on I'll do that." Rather, admissions officers want to read, "I learned that I should always get buy-in from the whole team, and I put this lesson to use several months later when another project came along..."
That's the key -- not what the failure was, but what you learned, and how you applied that lesson later on!
We write about failure essays and provide examples in my book, Your MBA Game Plan
. Pick up a copy if you think you need more help!