I tried it once when I was 6. My cousin (10 at the time) overheard me and jumped into the bathroom with a Jason mask on. (It was Halloween)Mason said:http://en.wikipedia.org/wiki/Bloody_Mary_(folklore)
I don't feel like reading the article, but when I was a kid we were all afraid that if you turn off all the lights in the bathroom (has to be pitch black) and say Bloody Mary three times, you die. To this day I've never tried it.