Inside, there was a check for a correction I sent him some months ago. In fact, it was not really a correction; it was more like a comment. And it was so obvious (he even said that) that he just sent $0.32, instead of the usual $2.56. But hey, who cares? I’ve got Knuth’s autograph now

Perhaps I should set as one of my goals to find a proper error, so that I can receive a $2.56 check By the way, the errata of the Concrete Mathematics is available online and this particular omission is documented as follows:

page 338, line 2 from the bottom
change “for $z$” to “for $z$ and multiplying by $a$”