Okay, I think I figured out why this happens.

I changed the code yesterday so that, when a post is hidden, the topic recounts its post counts to ignore the newly hidden posts.

However, the pagination system expects hidden posts to be reflected in those counts because hidden posts actually take up a spot on the page.

Thanks for bringing this to my attention. I'll think of a fix.

(I temporarily turned off post hiding til I fix this to avoid further issues)