I Don't Think That Word Means What You Think It Means
When one* of my answers on a test was marked wrong, I asked the prof why, and his answer defined a term differently than I had remembered it. I gathered up some internet definitions (Merriam-Webster, Wikipedia, some class notes) and sent them to him. He dismissed them as opinions of non-experts. Dog worrying a bone type that I am, I spent a couple of hours in the library and found the book in which the guy who invented the terms defined them. Somebody was mistaken, and it wasn't your less than humble correspondent.
Moral dilemma: does my obligation as a scientist to spread "the truth" trump my humane responsibility to further harass the prof who has enough trouble with a couple of elderly PhD's in his class? I don't care about the grade.
*One of many - but I usually understood why the others were wrong.