Thatcherization (or Thatcherisation if you're English) is the strange way in which people don't notice massively distorted faces, as long as they're upside down. Thatcherization received its name when its discoverer, Peter Thompson, demonstrated the phenomenon using a picture of Margaret Thatcher.
Humans are built to notice fairly subtle differences in the faces of other humans, but there are plenty of times when facial recognition lets us down. In fact, there are some experiments which demonstrate that people in ordinary interactions — like asking for forms to fill out — don't notice if the person they're talking to is replaced by a completely different person mid-conversation. Most of those failures are chalked up to lack of attention. But when a person focuses on a face, it's assumed that he or she will notice the details.
But when a faces is upside down, people have a more tenuous grasp of facial anatomy. The most common Thatcherization techniques are inverting the eyes and the mouth (so that they are right side up in an upside down face). It doesn't matter if the eyes of a photo have eyebrows where the cheekbones should be, the casual observer won't notice. Even when people are let in on the difference, the face doesn't register the same strangeness that it should.
Scientists think Thatcherization works because of lack of experience. People rarely see inverted faces, and so they aren't able to compare the way the eyes look in a Thatcherized face vis-à-vis a normal face. It's also harder to detect facial expressions that are inverted, so the social cues that people normally read automatically aren't there. Whatever the reason, it's funny to think that the human face can be made so strange with a simple flip.