When I'm on the meta site and view my achievements drop down at the head of the page:
The links contain relative URLs.
<a href="/questions/..."> ... </a>
This causes them all to point to Meta Stack Overflow (since this is the only place the new top-bar is implemented). Even achievements on other Stack Exchange sites.
http://currentsite/link-to-the-question-or-answer
, so cross-domain links break and it takes you to the question/answer with the same ID on the current site (MSO in this case)