Found this one pretty interesting (hope it hasn't been posted before):
Your brain won't let you see...
Someone explains the phenomenon like this:
In this optical illusion, the black dot in the center of your vision should always appear. But the black dots around it seem to appear and disappear. That’s because humans have pretty bad peripheral vision. If you focus on a word in the center of this line you’ll probably see it clearly. But if you try to read the words at either end without moving your eyes, they most likely look blurry. As a result, the brain has to make its best guess about what’s most likely to be going on in the fuzzy periphery — and fill in the mental image accordingly.
In other words, our brain just auto-completes what it perceives as a pattern, even though we know what's going on - our consciousness can't override this 'pattern recognition' system! Or so I understand it.