The horror mirror is a device that is meant to impact the user psychologically throughout an extended period of time.
Before the user goes to bed at night, he or she typically does some personal hygienic activities in front of the mirror, such as brushing teeth or removing makeup. The first interaction will be very nonchalant. In a split second the user would see their image depicted in the mirror with a small flaw. When the user sees this flaw and takes the double take, the flaw would be gone. The whole idea is that the user does not think anything of the flaw at first. As time progresses, the “flashes” of imagery get worse and worse, and the time displayed get longer and longer.

More information and rationale: DesignHorror