I think Douglas Adams was just trying to be humorous, but I can't be sure.

On the other hand, Ellie Arroway got the math wrong in the 1997 film version of Contact when she tells Palmer Joss how many planets with intelligent life could be out there in the Milky Way galaxy:

http://www.badastronomy.com/bad/movies/contact.html

Bad:
Ellie says ``You know, there are 400 billion stars out there, just in our Galaxy alone. If only one out of a million of those has planets, and one out of a million of those have life, and just one out of a million of those has intelligent life, there would be literally millions of civilizations out there."

Good:
Oops! A million times a million times a million is a quintillion, which is a lot bigger than 400 billion! She meant to say that there are 400 billion stars in our galaxy, and a hundred billion galaxies like our own. Even then, that's only 40,000 civilizations in the entire Universe! That's not very many. Even a pessimistic estimate would be a lot bigger than that!
Note (added May 22, 2003): Speaking of oops, I made either an error or a typo in this paragraph when I originally wrote it; I said 10,000 civilizations instead of 40,000. The latter is the correct number, and I thank the astronomy class at Emma Willard School in Troy, NY for catching me in that mistake!