I had a friend in 7th Form Calculus who technically failed her midterm, despite getting every merit and excellence question correct.
Our Teacher was a legend, so he showed this to us and explained:
Under the marking schedule, you had to get 2 specific Achievement questions correct, in order to get Achieved. To get Merit, you had to get the Merit questions AND Achievement. To get Excellence, you had to get the Excellence questions, Merit, AND Achieved.
She missed ONE of the two specific questions, (she miss-read a x for a + ("Hey Laura, what's 2 x 3?" "6" "Then why did you write 5?" " :facepalm: ").
He managed to talk to the other markers, and her merit questions were "counted down" to achievement, but it took a lot of doing.