I am a "formula" teacher, when comes to grades, which means I pre-decide the formula of grade calculation before I start a class. This is because I tend to get very personal with things and I don't want my personal attitudes get in the way of fairness.
What happens from time to time, is that good students I have known for a semester fail the final exam. This troubles me. On one hand, I know they studied hard, enjoyed the class and understood the material well. On the other hand, they didn't perform in the final.
Exams are not perfect but they are regarded as common means of evaluation in modern education. I know relying on fair exam is actually only partially fair. But, relying on my personal impression of the students would never be fair. So I choose to rely on the exam scores and my formula even though I am really really bothered by that.