So, according to it's description, to get the Rudolf hat you need to "Accept an answer on a new question".
I have been awarded the hat for moving my accept on the question How do I break a stack of objects? from an old obsolete answer, to a new up to date one.
This isn't a new question and does feel a little bit of a cheating way to get the hat.
Anyway, now you all know so either (a) it can be fixed or (b) everyone can get themselves a free hat.