This was many years ago, so it's a bit hazy, It was something to do with process ID assignment of some root process by the OS.

It's hard to explain, but he didn't come off as arrogant. He was one of those rare legitimate experts who had a hand in the formation of things we take for granted today, knew it, and didn't take kindly to young whipper-snapper know-it-alls with their Linux laptops open and not paying attention during class challenging him on his accomplishments.

He's one of the fathers of the modern notion of the field of "computer science", before that it was EE, or Math, or some other field. His PhD thesis introduced core multi-process OS concepts used in every Operating System since then and basically invented the modern theory behind Virtual Memory.

He helped design Multics which was worked on by Thompson and Ritchie which was the template for their sequel project Unix, he co-founded CSNET which was the academic Computer Science network that's one of those networks in the "inter" part of the word "Internet" -- it was also the network that hosted one of the first experiments in free software distribution (and is still around "netlib").

Challenging him on these kinds of issues is like challenging Neil Armstrong or Buzz Aldrin on actually landing on the moon.

He worked insane hours, put an incredible amount of time and effort into education (something more professors could learn from), had a very well structured course that had been refined down to near perfection and always went out of his way to make time for students. I remember many meetings with him where he was absolutely exhausted looking, but he never rushed us out of his office.

He was very insistent that you put as much effort into his class as he did, and anything less was disrespectful. And that's how he ran it.

He was easily the best professor I had in all of my education, full-stop.