It's usually what happens when you use a collapsable menu. What happens is you have a page document of lets say, 1200 pixels. The box you collapsed was 100 pixels. Your browser still thinks the document is 1200 pixels, while javascript altered it so 100 pixels kinda "disappeared." That 100 pixels has to be made up from somewhere and it's at the bottom of the page. I know about this and it's not a major issue, but I'm trying to think of a way to stop it from happening, as long as it's not overly complicated.