Common mistakes to avoid when filling out the UPSC application form include inaccurately entering personal details, such as name and date of birth. Errors in selecting the preferred examination centers or optional subjects can lead to complications. Candidates should ensure they meet eligibility criteria and provide complete information, including required documents. Furthermore, neglecting to review the form for consistency can result in discrepancies. Submitting the application before the deadline and confirming the receipt are also critical to prevent last-minute issues.