OK, so your main character's been through some rough stuff. Maybe's he's being bothered by ghosts, his own insanity, an Eldritch Abomination, or some other evil. He goes into the bathroom, turns on the sink, washes his face looks up at the mirror and.....BAM! Instead of his reflection, he sees some other face, and cue the Scare Note/Psycho Strings. Their are two types of the Different Face trope:
The Foreign Face: when an entirely different person replaces the character's reflecion
The Altered Reflection: when it's the characters face, but changed in some distubing way.
Not to be confused with a Mirror Scare, in which another person's reflection is added to the mirror
- Dr. Stein in Soul Eater, post-Kishin resurrection, sees himself in the mirror with the three-eyed Kishin symbol replacing his face
(Altered Face type)