That's from Mostly Harmless, 1992, so newer than Rick Cook's version.

But also sufficiently different that I have no doubt a lot of people have independently coined some variant or other. There's also the decades older (sometimes attributed to Einstein, but there appears to be no evidence that he said it) "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe." It seems sayings about the extent of human stupidity are quite widespread in many variations.