Some mirrors might be possessed by creatures known as Parallel Demons. These creatures travel between parallel universes through mirrors. To do this, they take the form of anyone who comes in front of the mirror. They can mimic the person's movements so well it's hard to tell they are not reflections. If the person gets close enough, the Parallel Demon will reach out of the mirror and grab them, then drag them through the mirror to their world. It will then jump through the mirror, trapping the one it was reflecting in their universe. This universe looks and feels just like the universe they left except for the fact they were not there before. The Parallel Demon will then live their life as if they were the person they reflected and not many will notice anything different.