Biomeg has it right, it's not realy an "error", it's just the way it displays when the threads get long enough due to how the coding that integrates everything work.
At this time there's no way I know to change it and keep the code the same across the entire site.
I know it's a little wierd, but I got used to it pretty quickly. If it's to bothersome you can use biomeg's way, or bookmark this URL
(You'll want to hilight the link, copy it, and past it into your browser, not click on it from here)
This is just the forum itself, so you won't see any of the double lines, but of course since it's JUST the forum you won't be able to navigate to any other part of the site.