HEY IT HAPPENED WITH ME IN AIEEE.IT WAS QUESTIOn NO 24. i had already marked 25,26,27 and 28, then i did 24th and by mistake i marked the answer in 29th. it was a really easy ques.but i lost 1.so u see it happens. one has to be very careful. papers are not manually checked so there is no point of cancellation. all the best 4 ur other exams. bye.