The system maintenance and troubleshooting utility "MainMenu" may cause a problem with Expose in Snow Leopard where a phantom window will appear when Expose is invoked. When highlighted, this will look like a large blue square next to any other window you may have open that's showing in the Expose view.
This does not appear to cause any other problematic behavior, but if you are seeing this blue window in Expose then try quitting MainMenu to see if it goes away. It's entirely possible another utility or application could cause this problem as well, so if you do not have MainMenu installed, try removing or disabling other system add-ons and menubar additions to see if that helps.
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