Sometimes, when a question gets deleted, not all answers of that question get automatically deleted too. Provided that you can see the deleted question, those answers can still be voted on/..., just like normal answers. This is inconsistent and should be prevented.
Most of the examples seem to fall into two categories:
Questions deleted for being spam/by moderators (just a guess, those questions have no "deleted by" and no record of their deletion in their revision history. Probably that's a bug on it's own):
However those questions got deleted, this action should probably also delete the answers to that question.
I would guess the reason for this is some side effect of the "can still post answer after question is closed" grace period (I seem to remember there was such a thing). This grace window should not apply to deleted questions or the posted answers should immediately be marked deleted.
In the latest data dump there were all together >4000 such "undead" answers. To find them the following query can be used:
select a_parentid as "question id", count(a_id) as "number of not deleted answers" from ( select answers.id as a_id, answers.parentid as a_parentid, questions.id as q_id from posts as answers left outer join posts as questions on answers.parentid = questions.id where answers.posttypeid = 2 ) as subquery where q_id is NULL group by a_parentid;
(The query for the real database would probably be simpler since there it should be possible to directly ask if a given question is deleted)