Form letters from authors make sense. I remember one time I received a yellow postcard back from Stephen King that I believe was addressed: Dear Constant Reader. And it basically explained that while earlier in his career he actually tried to respond to as many fan letters as he could, he figured he could either write letters or write books. And because I was a fan, the postcard told me, he was fairly certain I would rather he be spending his time writing books. It was still really freaking cool to receive. I’m fairly certain I have it still, somewhere around here.
But io9 has published a form letter from Robert A. Heinlein (snippet above), in which he could check any number of responses, including “You say that you have enjoyed my stories for years. Why did you wait until you disliked one story before writing to me?”
Considering some of these simply make reference to articles or other books, I can just see Heinlein tallying up his letters over a certain period of time and deciding that these twenty-one responses should be the ones to fit on a single typed page. The person who received this one, which you can see by clicking through to io9, obviously left the author speechless. Because nothing was checked.