This is a popular legend that you may remember from your childhood. The Mary Worth story--also known by such names as Bloody Mary and Mary Margaret--is popular at sleepovers. As the story goes, a beautiful young girl named Mary Worth suffered some sort of terrible, disfiguring accident (or occasionally the wounds are inflicted purposely by a jealous party). From then on, other people shun her due to her ugly face. In some versions she becomes a witch.
Now for the scary part. Supposedly if you say Mary Worth's name three (or five, or ten... it varies) times while looking into the mirror, Mary Worth will appear and scratch your face off or kill you. She is exacting a hideous revenge on the undeformed people who made fun of her in life.
The great Clive Barker movie Candyman is based on this sort of legend.
FAQ EXTRA Here is an interesting article about this type of urban legend.
FAQ EXTRA Another reader has sent in her interesting observations about the legend.
FAQ EXTRA A very bizarre spinoff of the Bloody Mary legend.
FAQ EXTRA One reader's experience playing the game.
FAQ EXTRA A page about the Bloody Mary legend
on Mythology Web.
This page (http://www.ghosts.org/faq/4-3.html) last updated April 13, 2005.