This is probably the trippiest thing I've ever seen! Some call it science ... I call it witchcraft! And do you know how we tell if it's a witch? "We build a bridge out of her!" .... ... ... you know what? Close enough.

From the science website that has words.

Like many interesting scientific discoveries, this one was an accident. Sean Murphy, an undergraduate student, was working alone in the lab on a set of faces for one of his experiments. He aligned a set of faces at the eyes and started to skim through them. After a few seconds, he noticed that some of the faces began to appear highly deformed and grotesque. He looked at the especially ugly faces individually, but each of them appeared normal or even attractive. We called it the “Flashed Face Distortion Effect” and wanted to share it with the world, so we put it on YouTube.