ambiguity
I took the biology semester of my mandatory science requirement in…I think it was my sophomore year of college. One short quiz was multiple choice, with five questions and five possible answers, not counting "none of the above". One question clearly had A as its answer. One question clearly had D. One question clearly had E. The remaining two questions were ambiguous. Both could correctly be answered with either answer B or C. I answered B for one of the questions and C for the other, and I turned my paper in.
I got them wrong, of course. The correct answer for both was, in fact, "B or C".