A favorite Knuth story to illustrate: He was re-writing the 8080 code for the processor that ran our new Alphatype CRS Phototypesetter, since the manufacturer's code wasn't suitable. Real-time, 7 levels of simultaneous interrupt stuff, since it controlled two stepper motors that moved the lens around the photographic paper, plus four custom font outline-tracing S100 cards that flashed vertical segments of characters at just the right moment on the CRT that the lens then focused onto the paper. No debugability other than a 4x4 keyboard and a 15(?) character display. I was responsible for the code on the other end of the RS232 port that connected it to the DEC mainframe, sending down character outlines on the fly to the very limited memory on the 8080, along with the properly-sorted character placement info ultimately from TeX.

Anyway, we'd been doing morning, noon, and night debugging sessions together for a number of days. On maybe the 3rd morning, I arrived at his office and was speaking with his secretary of many years, Phyllis Winkler, when Knuth came in. He handed her a stack of maybe a dozen or two pages, saying "Here's a paper I wrote; please type it in." When? When did he write it? It's still a mystery to me.