It's happened to me again, and I'm severely pissed now.
A user (with a rather high reputation, about seven thousand or so) just posted a question about a probability problem (something with people and shoes). I miss-understood the question, and posted a rather short answer. It was quickly pointed out to me via a comment that I had miss-understood the question, and I set out to solve the actual problem that was being asked, not what I had initially assumed the question to be.
I spent about 20 minutes fixing my answer - might even have been more - only to find out when I was finished that the user had deleted his own question in the mean time. So in effect, I just completely wasted nearly half an hour of my time, just because somebody was unable to make up his mind beforehand whether to post a question or not.
I believe that there should be some way to punish a user for that sort of rude behaviour. Ideally, I'd like to be able to put a user on a private blacklist, so that further questions of that user won't show up when I click "new questions". Or maybe there could be a cost in reputation for posting and then deleting a question - say, 100 reputation points or something like that.