The old parlor trick of staring at yourself in a mirror until your face starts to change has now been studied scientifically. Giovanni Caputo led a study in which 50 people tried the trick and reported their reactions.
At the end of a 10 min session of mirror gazing, the participant was asked to write what he or she saw in the mirror. The descriptions differed greatly across individuals and included: (a) huge deformations of one’s own face (reported by 66% of the fifty participants); (b) a parent’s face with traits changed (18%), of whom 8% were still alive and 10% were deceased; (c) an unknown person (28%); (d) an archetypal face, such as that of an old woman, a child, or a portrait of an ancestor (28%); (e) an animal face such as that of a cat, pig, or lion (18%); (f ) fantastical and monstrous beings (48%).
Have you ever tried this? The effect can be really creepy, like this video example from Lasse Gjertsen (in Norwegain, but may sound NSFW). The entire article is available as a pdf, or you can read a summary at Mind Hacks. Link -via Metafilter
To those who say they can't see anything - stop staring and relax.
I couldn't even pass a mirror in that state, things would reach out at me.