I've seen this bug (many times), as well, and it's a major pain.
For some reason (sql crashes, maybe?) the counters & pagination get off. The workaround I've been using, and it's a total kludge, is to move the entire thread to a another forum, use the 'moderate' function to 'split posts' -- select all of the posts one by one -- and move them back to the original forum under the same or slightly different title. It spits out a heart-stopping erorr, but it works.
Then I go back to the 'temporary' forum and delete the old topic title, which will now have 0 posts in it.
Nice kludge, eh?
If anyone has a script to fix this, I'd appreciate it. A Lot.