<p>I think it's up to you.  You can (and I have) delete your answer if you don't think it's relevant or contributes to the thread.  </p>

<p>On the other hand, you can also modify your answer to put a short note at the top:  </p>

<blockquote>
  <p><em>I understood the question to be asking ____.  In light of that, I provided the answer below.  However, it now appears that the question is asking ____ instead, which my post does not address.</em>  </p>
</blockquote>

<p>This may be a reasonable strategy for a couple reasons.  Others may have the same understanding you originally had.  Your post adds to the clarity and value of the thread as a whole.  Likewise, a future reader may come across the thread looking for your answer based on the same misunderstanding of the question.  Moreover, someone might learn an additional fact serendipitously despite / because your answer is different.  </p>
