There's a glitch in the matrix.
Topic's first and last post number post are stored in the topics table. These two columns are responsible for proper paging of a topic.
If for some reason, the the last post number doesn't get updated properly, the paging is fubar'd (eg. "Page 5 of 4").
Example:
If the system has to send too many topic reply notification emails and therefore violates the maximum script exec time, the script gets suspended and cannot update these counters.
A Forum Resync fixes this issue.