I got a response from Donald Knuth (and Ron Graham) after I pointed out an error in Concrete Mathematics.

Knuth’s response came on a photocopy of my original letter and he has really small handwriting. I didn’t get a cheque for $2.56 because they already knew about the error. Of course I wouldn’t have cashed the cheque, I doubt if anyone would, I might have had it framed though.

Still, a big thrill to get a letter from one of the world’s top computer scientists. I was a starting maths teacher at the time.