My forum users have named this the "Page Bug."
Here's what happens: Our forum has 30 posts per page in a thread. Somebody posts the 31st post (or 61st, 91st, etc) which should show up on a new page. However, the thread gets stuck. There's no "next page" number to click on. The post was successful, it's just not displayed on a new page.
At this point, I tried recounting, but it still did not appear.
The only way out of the mess is to make a new post. Suddenly, a new page appears and the 31st post is right where it should be, followed by the new 32nd post.
Sometimes the 32nd post doesn't work either, but that doesn't happen often.
I'm running v0.9.0.2 with a host of plugins. I've tried with no plugins and the default template but it still happens.