Cool Guys:
Even though the chances of a twin engine blade failure is so high the chances of hitting a flock of grease [sic] and knocking out both engines is probably less than 1 in 10 trillion, ...
A lot of people do take the easy way out and say that if the chance of one engine failing it 1 in a million then the chance of two engines failing is 1 in a trillion. That does not take into account that whatever improbable occurrence caused one failure is very likely to cause a second failure at the same time. (As in the flock of geese.)
The meltdown of the US financial derivatives market was not predicted because the SEC statistical experts evaluating the risk assumed that the chances of one mortgage defaulting was independent of the chance of another mortgage defaulting. Instead common causal factors hit them all at once.