It can be down to any of the following reasons:-
1) If someone has an extra wide image in their signature (or they have multiple images in their signature which, lined up side-by-side, are wider than about 600 pixels) then this can stretch the page ... this is why we have a few little rules about signatures
here, and avatars
here!. If everyone keeps their avatars to 100 pixels wide, and their signatures less than 600 pixels wide, then those users with smaller screen resolutions will not have any left/right scrolling issues when reading threads that you post in!
2) If someone's posted a big picture within a thread, this can stretch the page in a similar way to that described above;
3) The title of a topic can be fairly long, to be honest - it will just wrap around onto the next line. However, if someone types a load of characters together with no spaces (such as 'zzzzzzzzzzzzzzzzzzzzzz' - well, bigger than that example, but you get the idea) then it doesn't know where to break the line so it just stays on one line, thus stretching the page.
So - keep your avatars and signatures in check, don't type millions of characters together with no spaces - and our users with lower screen resolutions will not have any problems - yay!
