I'll be banking on this in the final exam

Back in Math 220 they showed us how to do proofs by induction, direct proofs, proofs by contradiction, and (my personal favourite) proof by cases. The thing they missed was proof by accident. This is when you've got only 2 minutes left in your number theory midterm and you have no idea how to do the last proof, so you write down some definitions and a few equations, and hope for some part marks. You hand it in as is, and in a weeks time when you get your exam back, you will find that you wrote a perfect proof, completely by mistake.