Have you ever attempted more questions than sought for ?

In our school or college exams. there would be choices for attempting the questions say out of 8 questions we are supposed to answer five or out of 5 questions we are supposed to answer three questions. But there are students who knows all the answers mentioned in each question and try to attempt more than the sought. That means they are over riding the prescribed rules and in that case what way the examiner would taking decision? Will he take best of the all the written answer, or simply reject the extras thus answered and in that case which answers he may discarded?