Have you ever heard about the Thatcher Effect? I just learned about it after seeing it in action -- and it freaked me out. First watch the video, then read the explanation.
Discovered by Psychology Professor Peter Thompson in 1980, the Thatcher Effect demonstrates that our brains "have, or learn, very specific abilities to process faces, their features and the information they convey" which are "'tuned to work best on upright faces,"as Open University explains:
So when a face is turned upside down some of these processes don't work so well, especially the ones that tell us about the spatial relationships between the main parts of the face: the mouth, eyes and nose. So when the face is upside down -- it's as if these abilities get turned off and so we don't spot the oddities.
It's named the Thatcher Effect because it was demonstrated by Peter Thompson using an image of Prime Minister Margaret Thatcher.