It's recommended that you watch this video twice. The first time, do as you're instructed and don't take your eyes off the cross in the center. The second time, however, look directly at the faces flashing in quick succession on either side of the screen. If you're like most people, you'll notice that the women you thought had hideous deformities while looking at the center of the screen are actually completely normal looking.
Researchers at the University of Queensland in Australia are calling this the "flashed face effect." How it works is that your brain focuses in on the main differences in each juxtaposition, thereby augmenting that difference to grotesque proportions. "If someone has a large jaw, it looks almost ogre-like," write the scientists. "If they have an especially large forehead, then it looks particularly bulbous."
The researchers say they don't yet know why the effect occurs, but they're attempting to find out now. In the meantime, hard as it may be, remember not to always trust your brain and eyes.