Not really a big deal, but was fun, during the second year of high school once I made a hidden C# program that would randomly open the CD lid and put it on some PCs of the lab and with some friends we watched the teachers get freaked out by the possessed computers (just to put things in perspective, today they still have windows XP, and their most recent laptops have windows 8 NOT 8.1 !!! And the "technician" says that they must not be updated wtf)

Eventually they couldn't find what was the problem and factory reset the computers

Also, we swapped Google chrome e IE icons and names on the desktop , pure evil