Common mistakes candidates make while preparing for the UPSC include underestimating the syllabus breadth and focusing too much on rote learning instead of understanding concepts. Many aspirants also tend to neglect current affairs, which are crucial for the examination. Time management is often overlooked, leading to last-minute cramming. Additionally, failing to analyze previous years' papers can result in poor exam preparation strategies. Lastly, not maintaining a balance between studying and self-care can hamper overall performance. Recognizing these pitfalls can significantly enhance candidates' chances of success.