Do you ever get that feeling you are being watched? Did you ever jump in horror to look behind you, only to realize you mistook a pile of clothes for a person? Did you ever stare in your face long enough so that it starts to seem unfamiliar? These incidents often will happen after watching a scary movie like Halloween, but there is actually a perfectly logical scientific reason behind it, that keeps you from going insane.
When you're in an environment that contains constant stimuli, your brain cancels it out. It adapts to steady, unchanging stimuli: the environment you're in will become faded and blurry. That's why you're so used to piles of mess on your desk, the constant humming of the AC unit, or even the sight of your own nose in the middle of your face! Did you know you're able to see it at all times, but your brain cancels it out so as to not interrupt your vision? The Troxler effect is what keeps you from overstimulation. It is what allows you to focus on the important things.
As you can infer from the examples above, this fading out of the brain happens over time. But when you look in the mirror without moving your eyes at all, the blurring happens in a matter of seconds. Your surroundings will become blurry until you blink, and then everything will reset. And that's why you might think you're seeing things when you take a long hard look at your face in the mirror.
Let's try a little experiment to demonstrate, shall we? Stare at the plus sign in the center of the image below for half a minute. The color stains should turn gray: