Wow, I was not expecting this article of mine to appear on Hacker News. The error I found was in volume 4a page 574 (as the memo line on the check shows). The solution to exercise 67 on that page says a particular circuit uses 6 ANDN gates, which I thought should be NAND gates. It gets a bit more complicated because Knuth was referring to the ANDN op code for MMIX, but there was still a mistake. (The other error I noticed was n choose k on page 824, but I checked the errata and it had already been found.)