This simple ruse can cause absolute looks of shock and anger on your intended victim, so make sure you choose the right target!
First, go to Flickr and find an image of a broken computer screen. This one is of a smashed LCD display with a big splash of black liquid crystal in one corner, perfect for tricking your victim into thinking that something terrible has occurred.
Save either image to your colleague’s Mac, close any open programs (probably best to save any work that needs saving) and go to Apple > System Preferences… Personal > Desktop and Screensaver and find the image that you saved to the computer and set it as the desktop background.
Finally, go to Apple > Dock > Turn Hiding On in order to hide the dock and any clue that the image is anything other than an image, then return to your own desk and await the consequences. You might even embellish the whole setup by laying the monitor face down on your victim’s desk or arranging some other supposed destruction!
Note that this trick is intended for Macs with LCD displays.
(Image via office.microsoft.com)