To this day I remember how outraged I was that on my final for Calculus 101 I derived from scratch an answer to an integration problem, then did the derivative and proved it was right on the final exam. Then the grader, upon seeing an answer different from the expected one marked it wrong.
I understand the grader was in a hurry, and the trig identity demonstrating that my answer was, in fact, equivalent to the standard one is not easy. But I had the right answer! And proved it was right, right there on the test!
I still remember the outrage. Over a question that did not matter then (I got an A+ in the course either way) or now.