What is big mistake done by UPSC aspirant who can not clear exam?

They all commit one mistake: They do not realize that Civil Service Examination is very different from school and college exams. CSE is not a test of your knowledge but a test of your overall comptence and skills to deal with challenges that one has to face as an administrator. Secondly, mugged up answers work well with college exams but the UPSC examiners would probably frown upon such answers as they expect innovation and originality.