I think I've worked it out. There are 25 posts on page 12 (replies 275-299, I think), as there should be. The missing posts are on page 13 (300-324). Due to the topic tracker bug or something, some posts were added to the thread, but the post count wasn't updated.
So the thread thinks it only has 296 posts, so it only needs 10 pages. Everything on page 13 is unaccessable, unless you play around with the URL (eg. going to page 12 and change the "start=275" to "start=300" should get you to page 13).
Maybe there is some way to manually set the post count. If not, this will happen at the beginning of every new page.
And while you're at it - this could very well be the same problem groza and I had in our respective games in Sicily. Care to check those too?
mole is now sleeping with the fishes.