Eliezer Yudkowsky:

There are large numbers of embarrassing people who believe in flying saucers, but this cannot possibly be Bayesian evidence against the presence of aliens, unless you believe that aliens would suppress flying-saucer cults, so that we are less likely to see flying-saucer cults if aliens exist than if they do not exist.

If your head is not yet nodding, see Eliezer’s “Intuitive Explanation of Bayes’ Theorem.”