Partway through the latest Medal of Honor game, you nearly die in an ambush. Your AI buddy saves you, then helps you to your feet. It should be a poignant moment; instead it's chilling, because his eyes are completely lifeless.
Or, you're playing Mass Effect 2, engaged in a serious conversation with one of your crewmates. Except that you can't take it seriously, because you keep staring at his teeth. There's something terribly wrong with his teeth.
Welcome to the Uncanny Valley.