1. Depending only on offline material - In the era of internet, if you aren’t aware of the latest development on web related to ias preparation , you might feel left out in the race.
2. Ignoring current affairs - Ira Singhal, AIR 1, 2014, has said in her interview that ignoring current affairs was the biggest mistake she committed during her preparation. Look at the previous question papers. Most of the questions in prelims and mains are centered around current affairs.
3. Failing to revise - If you fail to revise, there is no use in preparation at all. You may read 24*7*365 days, but what you recollect on the eve of the exam determines your performance. Hence, reading without revision = NOT READING AT ALL.
4. Preparing without a plan - UPSC is marathon-like race. Without a proper study plan, monthly targets, periodic assessment, clearing it is impossible.
5. Sourcing too many materials - Every day a new book is released in the market under the name "MUST READ FOR EVERY IAS ASPIRANT" / "SECRET TO TOP 2016" etc..Be wise. Be selective in buying materials.
6. Preparing without looking into previous question papers - This is the biggest mistake in my view. Unless you have seen the pattern of questions asked in the previous years, you will not be able to guide your own preparation. It s just like starting on a journey without knowing about the destination! Result could only be an accident!
7. No writing practice - Failing to write whiling preparing for mains, is a mistake which every candidate realizes after the results come out. For mains "writing practice" is the most important.
8. Selecting on optional- Selecting a wrong optional is another commonly committed mistake.
Sincerely hoping none of you to commit the above,
All the best.