Had a call from a friend "My mouse isn't working properly." The mouse would `single left click' OK, but `double left click' wouldn't work. I eventually got to the mouse controls by using the keyboard to find that a visiting grandson had moved the double click speed control slider all the way to the fastest side then pressed the apply button. There was no way thereafter to be able to double click fast enough to reverse the setting. Not really a practical joke, but it would have made a really good one.