Here's a handy example discussion of related conjunction issues from the Project Cyclops report:

We have outlined the development of technologically competent life on Earth as a succession of steps to each of [which] we must assign an a priori probability less than unity. The probability of the entire sequence occurring is the product of the individual (conditional) probabilities. As we study the chain of events in greater detail we may become aware of more and more apparently independent or only slightly correlated steps. As this happens, the a priori probability of the entire sequence approaches zero, and we are apt to conclude that, although life indeed exists here, the probability of its occurrence elsewhere is vanishingly small.

The trouble with this reasoning is that it neglects alternate routes that converge to the same (or almost the same) end result. We are reminded of the old proof that everyone has only an infinitesimal chance of existing. One must assign a fairly small probability to one's parents and all one's grandparents and (great)^n-grandparents having met and mated. Also one must assign a probability on the order of 2^-46 to the exact pairing of chromosomes arising from any particular mating. When the probabilities of all these independent events that led to a particular person are multiplied, the result quickly approaches zero. This is all true. Yet here we all are. The [explanation] is that, if an entirely different set of matings and fertilizations had occurred, none of "us" would exist, but a statistically indistinguishable generation would have been born, and life would have gone on much the same.