The OMR sheet is used when the examination format possesses multiple-choice questions. Sometimes the questions and answer options or bubbles are provided on the same sheet of paper and sometimes there is a different sheet that contains the questions and the OMR answer sheet is different. Every examination is different and has its own set of rules. Many students or candidates ignore the instructions in order to finish the paper fast. However, this is where they make a mistake.
One of the mistakes that many people make is answering more MCQ questions than required. For instance, there are 100 questions in total and you are supposed to answer only 60. If you have not read the instructions, you may answer more than 60; say 75, which is more than required. While there is no negative marking for answering more questions, this could affect your scores in a negative way.
The OMR sheet reader would be set to check 60 answers and would read the first 60 and ignore the other 15, even if they are correct. So, if you have made mistakes in the first 60 questions that you have answered and the other 15 are all correct, you are set to lose a lot of marks. Also, you waste a lot of time while you could have concentrated on answering questions that you were completely sure of.
So, you see, although more MCQ answered than required is really not a crime, it can cause you to score less than you deserve. Hence, always read the instructions and answer those questions that you are completely sure of.