Either the person had WoW set to use a non-widescreen resolution on a widescreen monitor, or they were using ButtonFacade with that rectangle skin.
/edit: ah, here it is:
http://www.wowinterface.com/download...Rectangle.html
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh