Buying every book and newspaper.
Usually toppers from other fields treat UPSC like any Other Exam.
Not following the routine.
Piling up too much sources and notes.
Skipping the past year questions.
Not taking NCERT seriously.
Not having a study plan.
Loosing out of patience.
Thinking that studying 24/7 will clear the exam.
Each major point is explained in comments below.
Meditate daily. Give yourself time if you are feeling low. Don't get too hard on yourself. Unhealthy mind won't help you in clearing the Exam.