[wespiser_2018]

If you ever try to get a knuth check, you'll receive back a printed copy of your email, along with his hand written notes and a response. Really cool!

[tjr]

I once received such a letter, in which Knuth explained how my bug report was in fact mistaken. I wrote back to him (on the same printout paper) to thank him for his reply, and included a check for $2.56. He cashed it!

[utopcell]

(-:

[Tomte]

I got back a comic, and another time a T-Shirt (with the MMIX instruction set).