hello,
I hope this bug wasn't reported before, I search in that forum but didn't found it.
When a forum has some redirection topics (e.g "Moved: Topic-that-didn't-belong-there") in it, they are not counted in the total number of topic posted in that forum (and that's ok).
Now imagine that you have a forum with 20 topics + one "moved:stuff" topic among it, and that the "Topics per page default" is set to 20 ( a total of 21 "entries")
You should have 2 pages for that forum : The 1st one with 20 entries (19 topics + 1 "moved" topic) and the 2nd one with only one entry: the 20th (the oldest) topic.
But instead there's only one page created, so the oldest topic can't be seen.
And off course, when the "Topics per page default" is set to N, then it happens every (1,2,3...)*N topics.
Anyway, as it only affects the oldest topic(s) only when a specific number of topics are posted I don't think it's an critical bug 
(using punbb 1.2.12)