Your brain makes imaginary monsters when you stare in a mirror

You know in horror movies when a person looks in the mirror and there's a killer or a monster or a ghost standing behind them? Well brain sure does. Often, if you look into a mirror for a few minutes at a time, you're likely to see a monster of some kind. Of course it's only an optical illusion.

Your brain only has the power to focus on so many things at once. All the senses are affected by this but sight especially. The mind tunes out everything that it isn't directly focusing on. In fact, the Troxler Effect is when you stare at one thing for a long enough time that other things in your vision fade out.

In an experiment with people staring into a mirror, they saw deformations of their own face, their parent's face with features changed, an animal face or a monstrous being.

(Source)