When you missed something important!
Do you realize the importance of instructions given at the head of an exam paper? This thread highlights that point and brings out an analogy between paying attention to exam instructions and other general areas of life as well.
Remember the instructions written at the top of the question papers while writing the exams? Those instructions were general in nature like the answers must be to the point and marks will be deducted for bad/illegible handwriting. In many sections of the question paper, there were options provided to answer a specific number of questions out of the lot. In many cases, a few of my friends never followed the instructions properly and tried to answer all of the questions instead of the specific number mentioned in the instructions which resulted in wastage of time. That time could have been devoted to other sections. Actually, I found not following properly what others are saying or any instruction is a common trait. Many people do not follow properly. Imagine a situation when you bought tomatoes when your mother asked you to bought potatoes. A little alertness could have altered the situation. Or think of the case when somebody told a person to bring the PAN card and that person confidently carried the Voter I-card thinking that would solve the purpose.
In the examination hall, you will find many students writing the exams just after receiving the question papers because they found many questions common. They do not pay any heed to what the invigilator is saying as if they will forget the answers if they do not write immediately. Life is not any examination, be alert and follow what others are saying or written somewhere as an instruction. Otherwise, you may be missing something very important.