I did rather poorly in an ethics class because I realized moral relativism was rather obviously right and any form of realism logically indefensible (without an axiom - if you admit you have an unprovable assumption underlying your moral framework, then it's morally real-ish, but it isn't totally objectively correct, because of that assumption). There is also a real risk of relying on concepts that the reader is not familiar with, but that's usually pretty easy to screen, especially if you have a friend to read a draft.

The thing is, when I say, "rather poorly," I mean a B (possibly +), and this was at a top public university. In a math class (having not taken math in five years), I realized shortly before an exam that I had been taking derivatives when I should have been integrating. This would have had a rather more pronounced effect on my grade had I not caught it.