I don’t know if these are deliberate, if nobody ever noticed, or if someone actually thought they were a good idea.

• Most Windows versions have a “system menu” in the top left of typical windows (aside for dialogs), formerly shown by a minus sign, and, as of Windows 95, with an icon representing the window. Single-clicking it opens the menu, double-clicking performs its default action: closing the window. (When they added the close button — the ‘x’ — in Windows 95, they probably decided to leave the action intact for legacy reasons.) The menu can also be triggered through right-clicking anywhere in the title bar, but I digress. In Windows Vista, for the sake of I’m-not-actually-sure-what-exactly, Explorer windows (but not Internet Explorer windows, nor other applications’ windows!) have no such icon any more, nor do they have a label for the window title, but they still have the menu! So by clicking in the imaginary, invisible 16×16 square that would include an icon but doesn’t, you can still open the menu, and by double-clicking, you can still close the window.
• Much more confusingly, the area right underneath the notification area icons, while completely transparent, also performs an action: double-clicking it opens Windows Defender. Its contextual menu suggests no such behaviour, neither by default nor otherwise. Yet I can consistently reproduce it, at least right now. The icons are Brightness (from Apple’s Boot Camp drivers), Skype and Gaim, and double-clicking anywhere below them opens it. Huh.

I’m almost inclined to think the latter is more of an easter egg than anything, but still…