These URLs work in a vote to close » duplicate dialog:
http://superuser.com/questions/231273/what-are-the-windows-a-and-b-drives-used-forhttp://superuser.com/questions/231273/what-are-the-windows-a-and-b-drives-used-for/233284#233284http://superuser.com/q/231273/22317
This one doesn't:
http://superuser.com/a/233284/22317
While this URL is redirected to the second URL in the first list when navigated to, it does not contain the question ID, only the answer ID. This is a technical detail that should not impact the user. It does, unfortunately:
