Common mistakes that candidates make while preparing for UPSC include inadequate time management, not understanding the syllabus thoroughly, and ignoring current affairs. Many aspirants also focus excessively on rote memorization rather than grasping concepts deeply, leading to challenges in applying knowledge during exams. Additionally, neglecting physical and mental well-being, overly relying on a single source, and failing to practice answer writing can hinder progress. A balanced, strategic approach to preparation that includes review and practice is crucial for success.