When I was a kid, we'd play this game at Sleepovers. I'm sure you're familiar with it. We'd turn all the lights off, then chant "Bloody Mary" into a mirror three times. As far as the version of the myth that we had heard, if your house was haunted by the right spirit, Mary's face would appear bleeding in the mirror.
It turns out we had it all wrong, but the Bloody Mary folklore does go back a long way.
Historically, the ritual encouraged young women to walk up a flight of stairs backwards while holding a candle and a hand mirror, in a darkened house. As they gazed into the mirror, they were supposed to be able to catch a view of their future husband's face. There was, however, a chance that they would see a skull (or the face of the Grim Reaper) instead, indicating that they were destined to die before they married.
Strangely enough, there is some scientific explanation to this. The Caputo Effect, is believed to be a consequence of sensory deprivation - causing the brain's facial-recognition system to misfire. One possible cause of the Caputo Effect is staring into a mirror in dimly lit environments. Caputo hallucinations include facial distortions such as melting or bleeding and the appearance of strange faces.