My school had a shared drive where anyone could create files and folders. There was one folder where people would put random things (bash scripts downloaded from the internet, pirated minecraft, etc.). I wrote a python program that would display a picture of a pineapple on your screen (no ability to minimise, or move other windows in front of it). I had python installed to one of those folders that only appears if you select "show protected operating system files". I later wrote a remote-control script that communicated by creating files on the network. I was able to control the mouse and keyboards, open applications, take screenshots, and monitor keypresses (I never got anyone to click on it without me telling them to though). Never got discovered for any of it...