Common mistakes candidates make while preparing for the UPSC exam include neglecting the syllabus and focusing too much on one subject at the expense of others. Overloading their study schedule without adequate breaks can lead to burnout. Additionally, some aspirants fail to practice mock tests, which are vital for gauging readiness. Inadequate analysis of previous years' papers and ignoring current affairs also hinder performance. Lastly, not being adaptable or open to feedback can prevent candidates from improving their strategies based on assessments of progress.