Take this trivial example and open it:
<html>
<body style="background-image:url(http://upload.wikimedia.org/wikipedia/commons/2/21/Mandel_zoom_00_mandelbrot_set.jpg);background-repeat: no-repeat; background-position: top center;">
<div style="width: 8000px; border: 3px solid red;">Demo</div>
</body>
</html>
The page is made so that the body has a top-centered background picture and a containing element which overflows window boundaries so there is horizontal scrolling (if you have a monitor wider than 8000px then you're really cool, please make the window smaller and refresh).
The problem is that for some reason the <body> doesn't stretch to contain the <div>. It just stays the same width as the viewport and the <div> overflows it. This in turn causes the background to be centered at the wrong place and crops it to the size of the viewport. Quite ugly when you scroll to the right.
I've already found a solution to this problem, but I'm wondering WHY this is so? It seems to be consistent across browsers too. But in my opinion this is quite counter-intuitive and basically plain wrong. The container element should be big enough to contain it's children - unless they are absolutely positioned of course in which case they don't participate in the layout calculations.