TI-83 graphing calculator in high school, around 1998. I would sit there in math class coding games in Basic. Ended up developing a reputation as the guy you went to if you needed a program to cheat on a math test.

The highlight of the entire endeavor was a class wherein the teacher announced that before a test, they'd be resetting the memory on everyone's graphing calculators, to prevent cheating. I wasn't planning to cheat, but I did have a few games I was working on, and I didn't want to lose them, so I wrote a program that emulated the graphing calculator's interface, and would let you go through all of the steps to reset the memory, including showing the Programs menu as being empty afterwards, while not actually resetting anything.

I showed this to the teacher just before the test (demonstrated "resetting the memory" with the program running, then demonstrated that the memory was in fact not reset), and he backed off from the compulsory reset policy in favor of the honor system, because he conceded that he wouldn't be able to verify that the memory was actually reset anyway. Made me feel like an absolute hero.

It's honestly funny because I learned the concepts in the math classes a lot better as a result of this - it took a very thorough understanding of how to use a concept to write a program to solve it for you.