There are two reports at Google Webmaster Help forums that Google Webmaster Tools has a display bug. Specifically, when you login and try to see your data, your favicon logo is taking up a nice portion of the screen.
Here is a screen shot from one of the webmasters posting an example of the issue:
It seems like a browser/caching issue but Google has not yet responded in the thread.
I personally cannot replicate it in Chrome running on a Mac but it does appear some are having this issue.
Forum discussion at Google Webmaster Help.