It happens in game threads sometimes too, so I assume it's any thread with a high volume or high rate of posts.
You can see the last page of the thread though, it's just that the page number is incorrect. So if you click the last page button, and it takes you to 158, but the website shows there is a page 159 and 160, ignore it. 158 is the last page and 159 and 160 don't actually exist yet. I don't know why this happens, but it does.
I don't know if it has something to do with deleted posts or something, but don't worry about it. If the website is only showing posts up to page 158, then there are only posts up to 158.