Here's a prank I played at a university. They were using Windows on their computers. So I took screenshot of the desktop deleted all the icons hide the toolbar and set the screenshot as desktop background
After few while one girl sat on that pc tried to click on the icons but failed she was clicking the mouse hard that actually made me laugh and finally left off thinking that the pc is broken .
After few days turns out they called up an it guy for repairing that pc but even he failed at recognizing that prank and formatted pc think its some kind of virus or something.