Just checking in this morning to say it is still happening for me. I guess it's pretty obvious now it's not browser related.
Technically, it is happening in every forum, you just don't notice it if a forum has only a handful of pages, or is set to only show a handful of pages.
It looks like whatever setting in the message board software that shortens the sequential list of page number links across the top of the page has been knocked out somehow.
Edit: Just in case you aren't sure what I mean, in General RPG Discussion, across the top I see Page 1 of 1384, and then there are jump links to every number from 1 to 1384, which stretches out the page so you have to horizontally scroll to see the right half of the regular message board screen. This is on the same line as the New Reply button.
In meta, I see Page 1 of 2, and then the numbers 1 and 2 are listed, so it isn't noticeable.