astroengine writes: "A newly discovered affliction has some doctors wondering if astronauts traveling to Mars could be blind by the time they got there. About one-third of the U.S. space station crew members return with impaired vision, a condition in which at least one case was permanent. The reason for the late discovery of this mysterious affliction is down to the lack of astronauts on active service coming forward — the risk of being grounded is after complaining of blurry vision is considered too great amongst active members of the astronaut corps."
Computers are unreliable, but humans are even more unreliable.
Any system which depends on human reliability is unreliable.