If you believe a clerical error led to you receiving an incorrectly low grade, this is a valid reason for appeal. Clerical errors involve misprinted results, confusion between students' work, or typos within assignments. However, clerical errors can also refer to assessors misplacing some of your work, or reading your answers incorrectly.
Being discriminated against because of your age, race, gender or other characteristics is usually cause for appealing a grade. If you have reason to believe that your grade was affected by anything other than your work, this could be cause for discrimination; this includes if you believe that the person assessing the work has a conflict of interest toward marking your assignment fairly.
Although different education systems will have different policies in place for mitigating circumstances, generally if there was a major incident occurring in your life during your assessment, it may be possible to successfully argue that this negatively impacted your work. Mitigating circumstances vary, but they normally include a death in the family or a serious medical problem; this can often include mental health issues.
If it appears that your work was marked under different criteria than you were originally told, this can be grounds for appealing a grade. It's important that you are always aware about what aspects of your knowledge affect your final grade, and if this deviates strongly from your expectations then this might enable you to successfully appeal your mark. For example: if you were originally told by your teacher that grammar would only constitute a minor part of your English exam, only to later find out that it counted for 50 percent.