Right now I am feeling slightly angry about this: A two-part question was posted and an answer to both parts added withoin less than an hour. Nothing happened for three weeks (no comments or request for further explanation, no additional answers. Then, a few hours ago, a first comment appears (which I could only answer a few hours later) and shortly after that a self-answer - for only one of the two parts. The asker has the right to accept whatever he likes, but I find this somewhat strange and don't know how to react ...
Accepting a late and partial self-answer when a full answer was never further discussed
14$\begingroup$ As I have commented before: Voting is a capricious thing. Drown your sorrows in the next question :-) $\endgroup$– robjohn ModNov 3, 2012 at 23:14
1$\begingroup$ I have a text file I call idiots.txt to keep track of such people. There are many types of abuse of these sites, and one cannot keep track of everybody involved. Also some delete their own questions. Anyway, see tea.mathoverflow.net/discussion/753/… for more detail, a bit off topic for this thread. Let me think: the summary is that I say you have every right to feel aggrieved. $\endgroup$– Will JagyNov 5, 2012 at 21:06
$\begingroup$ I see, the self-answer reached 4 downvotes and was self-deleted yesterday. $\endgroup$– Will JagyNov 5, 2012 at 21:41
$\begingroup$ @WillJagy: ... which even earned him a peer pressure badge :) $\endgroup$– Hagen von EitzenNov 6, 2012 at 16:24
2$\begingroup$ What's so bad about accepting their self-answer, anyways? Is it because the "accept" signal is not just a signal for the asker themselves? $\endgroup$– The_SympathizerNov 10, 2012 at 7:51
$\begingroup$ @mike4ty4 I have absolutely nothing against self-learning and then accepting self-answers. But in the case above there was absolute discussion silence in the thread for weeks after a full answer (with possible need for clarification) and then suddenly a partial and incomplete quick-accepted self-answer. Because of the incompleteness I fear the OP only betrayed himself. Actually, I'm not fully sure - we cannot force anybody to accept what "we" consider the best answer anyway, it is always their decision. $\endgroup$– Hagen von EitzenNov 10, 2012 at 8:14
This isn't really even strange behaviour by the community at large; it's just strange behaviour by one particular user. If it were the former, we could get together and discuss it here and try to agree on norms.
But this is a case that happens every once in a while, when one comes across users whose idea of interacting with the site is what one might see as... suboptimal. You can of course try to engage with them in the comments; they may realize they accepted their own answer prematurely and try to be more responsive to other responses in the future. Or it might turn out, as has happened to me before, that they just don't care to listen to your suggestions -- in that case, there's nothing to do but just shrug it off and move on, and perhaps avoid investing time into answering their questions in the future.