"Staring into a mirror in a dimly-lit room may eventually cause one to hallucinate. Facial features may appear to "melt" or disappear, and other hallucinations (such as faces) may appear. This phenomenon is believed to be a consequence of sensory deprivation—causing our brain's facial-recognition system to misfire.[6] Troxler's fading,[7] self-hypnosis, and the Caputo Effect have been posed as explanations for the Bloody Mary phenomena"
Source:
https://en.wikipedia.org/wiki/Bloody_Mary_(folklore)#Phenomenon_explained