Grading is often perceived as one of a teacher's most difficult tasks. Despite most teachers endeavoring to grade their students as objectively as possible, many students feel that they are subject to unfair grading. The aim of this study is to describe what it is about a teacher's grading that contributes to the perception of unfairness. This study used the critical incident technique, with 411 practicing teachers in Sweden contributing their own experiences of being assessed in a school context. The main findings from qualitative data analysis indicate that when teachers fail to follow guidelines of the current grading system, use undependable information, allow themselves to be influenced by irrelevant factors, or are ambiguous in their communication, the pupils perceived the grading process as unfair. The findings are analyzed and discussed in relation to a theoretical model of teachers' grading, validity and reliability, and three ethical principles.