Not quite a straight up math error but my high-school english class had a weekly 10 question spelling/definition quiz. Each question was worth "10%" if you got both the spelling and definition correct and "0%" if you got either wrong. I typically got all the definitions correct but very few of the spellings (a major weakness of mine) scored typically 20-30 "%". At the end of the semester these tests were weighted equally with the 2-3 essays and a aggregate reading comprehension score which I did very well on (typical grades of 98%). The result was me scoring "40%" in a class where I was one of the top participants. No amount of explanation from the principal, my family or me could change the teachers mind on the fairness of this.