I made a lot of mistakes in class 11th, which I later caught up with in class 12th. So, the mistakes mentioned below were made by me primarily in 11th standard

• Ignored NCERT textbooks and stayed miles away from them thinking that NCERT textbooks are for noobs
• Forgot that I myself was a noob (became overconfident)
• Wasted a lot of time in playing cricket
• Wasted a lot of time on Facebook and other social networking sites
• Wasted a lot of time in playing GTA games (San Andreas and Vice City)
• Ignored Inorganic Chemistry and got swayed away with the beauty of Maths and Physics
• Solved less number of problems
• Read less of theory in Physics and Maths
• Solved less problems in Chemistry
• Spent less time in understanding general organic chemistry (GOC)
• Relied on coaching teachers for learning
• Didn't take tests seriously - never did so called "forward analysis" (analyzing the mistakes committed in an exam)
• Didn't really read/solve problems from book and relied on coaching notes
• Ignored subjective problems in Maths
• Used calculator to while solving problems of Physical Chemistry
• Read too less theory in Waves and Sounds (Physics)
• Read too less theory in Thermodynamics (Chemistry)

Some people have mentioned about not making a schedule/timetable. Well, I too didn't follow any specific timetable. However, I feel that not following a timetable isn't a mistake. Timetables look good only in theory. They are hard to realize in practice and it is absolutely fine if you are unable to follow a timetable.