Disconcertion Mirror

In a public bathroom, a person stands at the basin washing their hands. Above the basin hangs a small mirror - it is rather battered and tired, and the silvering has failed in places, producing several irregular black blotches.

As they glance up at the mirror, they are horrified to see a large, gaping gash down their left cheek. Their hand automatically goes to their face, but they feel nothing wrong and, looking again, the gash has gone.

Some time later, another person stands washing their hands. As they check their hair in the mirror, for a split second their reflection's lips curl back to reveal teeth filed to points, and their bloodshot eyeballs pop forward. But only for the briefest of instants.

The Disconcertion Mirror is, of course, a high-resolution display. One of its black blotches hides a camera which is connected to a hidden computer that can perform fast image analysis, gaze detection and image generation. Normally, the screen simply displays a left-right reversed image of the face in front of it, until the computer judges the moment right to disconcert the viewer. Naturally, the camera also records the response of the victim, for the amusement of others at a later time.