There was a friend of mine in school who would leave space in his exam answer sheet if he didn't know the answer to a question. And when the teacher distributed the answer sheet, he would write the answer then & claim the teacher forgot to correct.

He's a rich businessman now

Usually Teachers cross out all the empty space in the end, but rarely do they cross out empty space between two answers.
Another trick was claiming counting mistake. English/Hindi teachers would fall prey

Another common technique was to solve a question twice, one at the beginning and at the end.

But the real genius were the ones who for a True-False question would write their 'T' such that it could be changed to an 'F' with minimal effort later.