The way these "should" work is if you were to start at the very first topic on page 1, "previous topic" would step you through every topic on the forum until you were at the last one. "Next topic" would take you from the last topic on the forum, to the first one on page 1.
Next seems to be working ok, but it looks like Previous is bugged a bit still.
There has never been an equal opinion from everyone on how these should work. Some thing previous should take you down through the last and not up, some vice-versa.
