There are plenty of issues to disagree over. I remember some argument over what issues were important in program efficiency. He was probably right about that. I was dismissive of the practicality of pure LISP with no extralogicals and no sequencing (no 'seq' or indexed iteration). I was probably right about that.

It didn't help that I was a bit of an arrogant twit at the time.

But, the key wasn't grading. The most important factor was his claiming, at the end of the semester, not to have received an important homework from me. I had thrown it out by then, so I couldn't prove I'd done it; and he gave me a zero on it.

This could have been accidental. But it never happened to me in any other class.