What's Going On, Man?
You are working on the site and then you notice that when you click through several pages the site shifts horizontally by several pixels and you wonder wtf is up. The answer is: scrollbar.
The shift happens when you click from page with little content (no vertical scrollbar) and land on a page with lots of content (vertical scrollbar appears). The problem becomes more apparent when you have some nifty-crifty scripts on the page that hide/show large piece of content and, depending on the state, the scrollbar appears and disappears.
Chances are you won't notice that in IE6 and IE7 because by default it got the "fix" already in place.
Here's a demo with a scripted button that hides/shows large chunk of text demonstrating page-shift problem.