The SE definition of "answered" question is: has an answer that is either accepted or has positive score. But the search parameter
isanswered appears to ignore the "accepted" clause: when set to "no" it still returns questions with an accepted answer, as long as it has 0 score.
Sample query to reproduce the bug (unless meta effect kicks in and the answers get upvotes).
By the way, it seems that
isanswered works only when
is:question is present, but I'm not complaining about that... yet.
Also, this parameter is not listed on the advanced search help page, but I'm not complaining about that either... yet.