This was reported to me today 

Mike, I think I found the reason.  Actually NoGoingBack mentioned it in the tweaks and UI thread.  Having an ignored person messes with the page count just like hiding signatures.  So I have ignored Jor-El and without seeing his posts, the thread was only 4 pages to me, rather than 5.   So going to the “new” post didn’t work and page 5 was non existent.

On the main forum page, the errors were probably because the thread was marked with a “new” when it shouldn’t have been.  I marked the entire forum as read, and those errors went away.