I wondered how you received a real check since Knuth has not been sending out real checks (as you have noted) for some time now, only to see that you explained it in the very first sentence and I had read over it, just like many people read over the error that you found in the first sentence of the book. Nice one! I wonder what the "Infinitely" was replaced with - probably not "Finitely" :)
I had received a check recently and wrote about it too https://news.ycombinator.com/item?id=49286258