I asked this question and got an answer from someone who misunderstood my question (see my comment). Should I downvote this answer, even though it was presumably my fault (I wasn't clear in the question)?
If the question is not clear, I would not down-vote somebody who is trying to answer the question. It would be better to comment first, asking what exactly the question means, but I would not punish somebody who is trying to answer it.
Clearly, if the answer is blatantly off, such as in the case somebody is asking about an implementation of
hook_menu() with an answer that explains how to use
hook_init(), then a down-vote could be deserved; the down-vote should also be removed once the answer's author notice the mistake, since the down-vote is not for the user, but for the given answer.
Down-votes are subjective, though; what somebody would down-vote could not be down-voted from somebody else.
Down-voting depends also from the context. It can happen that somebody misunderstand a question (which could be because the question is effectively ambiguous), but if the same user keeps answering with the same, wrong answer questions about the same topic asked by different users, then the situation is different. In the first case, a comment could be left; in the second case, a down-vote would be more appropriate, and it would say to the user "stop giving that wrong answer, and answer what the OP is asking." (At least, down-votes should make the user wonder why his answers get down-voted.)