When the
Go to last post (
) button is used, the thread display gets screwed up in that the top of it doesn't scroll up high enough to reach the forum navigation links, so the only way from a thread accessed in that way is to go to the main forum again through the link at the top of the site.
See this picture to understand what I mean:
http://img510.imageshack.us/img510/8983/exampleqv6.jpg