You like your eyes, right? We certainly like ours. Most of the time they work awfully well -- despite a massive design flaw that any first-year engineering student would have caught.
It's called the punctum caecum, an area at the back of the eye that the optic nerve passes through, obscuring the light-sensitive photoreceptor cells that make vision possible.
That blind spot also makes an awesome optical illusion possible. Richard Wiseman, who runs the Quirkology video channel on Youtube, has concocted a particularly striking demonstration of the human blind spot -- by making his own head disappear! Weirder yet, he's able to pass a black bar up and down within the blindspot and it still seems visible, because the human brain is capable of reconstructing such simple objects and filling in the gaps.
See -- or don't see -- for yourself:
How does it work? Because our eye is wired 'backwards' -- the optic nerves resting on top of the photoreceptors -- there's no way for the blind spot to be populated with light-detecting cells. It's pretty particular to mammals; in octopus eyeballs, for example, the photoreceptors sit on top of the optic nerve, negating the blind spot. We humans just have to make do the best we can with this gaping hole in our vision.
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