Tricky Statistics

The Atlantic features 5 seemingly counter-intuitive statistics stories.  My favorite:

Abraham is tasked with reviewing damaged planes coming back from sorties over Germany in the Second World War. He has to review the damage of the planes to see which areas must be protected even more.
Abraham finds that the fuselage and fuel system of returned planes are much more likely to be damaged by bullets or flak than the engines. What should he recommend to his superiors?

ANSWER: PROTECT THE PARTS THAT DON'T HAVE DAMAGE

Abraham Wald, a member of the Statistical Research Group at the time, saw this problem and made an unconventional suggestion that saved countless lives.
Don't arm the places that sustained the most damage on planes that came back. By virtue of the fact that these planes came back, these parts of the planes can sustain damage.
If an essential part of the plane comes back consistently undamaged, like the engines in the previous example, that's probably because all the planes with shot-up engines don't make it back.
Wald's memos on this situation -- in addition to being a remarkable historical statistical document -- shed additional light of the statistics developed during the Second World War that would go on to found the field of Operations Research.

The others are pretty interesting, too.