Why do some questions disappear?

I took time and thought to answer a question today only to have the whole thing disappear. The person who'd asked the original question then posted an updated version of the same question. I answered again, only to have it vanish into thin air once again.
I wasn't aware that people could erase their questions. Has this happened to anyone else? Am I the only one who finds this is a waste of time for those who bother to answer?
Keep an eye on this space, I'll see if I can make it disappear in the next hour.