Exams by their nature have many pitfalls and performance in exams is affected by factors such as ability, demeanour, study, preparation, and stress levels.
Exam takers can make mistakes in preparing their subject material.  Misread exam questions are a common problem.  Providing incorrect or incomplete answers are also common issues.  Running out of time is a frequent experience for exam takers.  Question selection, time allocation, and mark 'harvesting' can affect performance and the results achieved in exams.