Mischief Managed
In elementary school we had the Accelerated Reading (AR) program. You would read a book, take a test on the computer, and be awarded points based on how well you did. At the end of the year you could buy things at the book store with the points you accumulated.
I had just finished reading Harry Potter and the Prisoner of Azkaban and got a perfect score on the test. The computer was only supposed to allow you to take the test once but I figured out you could take that specific Harry Potter book unlimited times. I racked up so many points and was never found out.
Story credit: Reddit / saucecat2