Hemineglect is my personal favorite (boy it feels wrong saying favorite) example of this. Those with hemineglect are largely or completely unaware of the side of the universe contralesional to their brain damage. They can pick up a chair on one side of their body, put it on the other, and when you ask them where the chair is, they say "What chair?" To make matters worse, they also often insist that they have no deficit in function. It's astounding.