Don't know if it's a bug or not. Some of my users are facing this strange thing. Sometimes the header of the page disappear from the top and appear at the bottom of the page as you can see in the two screen attached.
I've asked and it seems that the problem face up only in Windows and not in FF for Mac OS X.
The header is located at the bottom of the document, and it moved upwards with CSS. If your users are seeing this it would tend to indicate that something is getting stuck and causing pages to load slowly - the header will appear at the bottom of the page until the CSS is fully loaded. If the problem rectified itself with a refresh, this would tend to indicate a slow server or a slow user connection to it.