This is one of the common mistake that candidates commit. Due to the haste and anxiety every one has the eagerness to open the test paper as quickly as possible and start solving it. At the beginning of the testpaper the major concentration is towards judging whether the question paper is easy or tough. So generally candidates skip the instruction page and jumpover to the questions to start solving as quickly as possible and to be in knowledge that whether they are comfortable with the paper. And as the candidates come across with an easy question then subsequently the next one and the next one and in the quest of quickly answering as many questions as possible, the just keep on postponing the reading of instructions. This psychology is the major reason for this blunder of answering ore questions that asked.
Moreover many a times, when the question paper has multiple sections the instructions are given at the beginning of each section. That means every instruction is not provided at the beginning of the question paper. Now while randomly jumping over from easy question to another easy question generally the instructions at the beginning of the section get missed. This is another cause of answering more than required questions.
Technically when you answer more than asked questions, there isn't any problem as the system reads first 'N' number of questions to evaluate. i.e. if there were 25 questions to be answered, out of 50 total questions and the candidate by mistake answered 35 questions then the software will pick first 25 solved questions by serial to evaluate. It will ignore the other 10 questions even if they are solved properly. No marks are deducted for extra solved questions. The only disadvantage that the candidate fares is that he has lost lot of time in solving questions that were not required to be solved or are not going to positively effect in any way.
This ignorance not only wastes the time but also increases the chances of negative marking. Because for example you marked 35 question instead of required 25 questions out of 50. Now not all 35 questions you had same surety. For some of the questions you would be extremely sure while for other few you might be taking chances. Now suppose if these questions for which you have taken chance are by default included in the first 25 to be evaluated and the remaining 10 which are left as surplus have been marked with surety then your evaluated part would have less chances of fetching you marks. Had you been cautious to mark only the 25 sure answers, there would have been least possibility to get any question wrong and thus get negative marks.